Homocysteine Testing

Number: 0763

Table Of Contents

Policy

Scope of Policy

This Clinical Policy Bulletin addresses homocysteine testing.

1. Medical Necessity

   Aetna considers homocysteine testing (measurements of plasma homocysteine) medically necessary for the following indications:

   • Assessment of borderline vitamin B12 deficiency, where the results will impact the member's management (see CPB 0536 - Vitamin B-12 Therapy); or  
   • Assessment of central retinal vein occlusion if member has a history of previous thrombosis, or family history of thrombosis, or if member is under the age of 56 years and does not have strong evidence of arteriosclerotic risk factors; or
   • Assessment of homocystinuria caused by cystathionine beta synthase deficiency (Note: For newborn screening, measurements of plasma homocysteine/total homocysteine are performed only when hypermethioninemia has been confirmed); or
   • Assessment of idiopathic venous thrombo-embolism, recurrent venous thrombo-embolism, thrombosis occurring at a young age (i.e., less than 45 years of age), or thrombosis at an unusual site.

2. Experimental, Investigational, or Unproven

   Aetna considers homocysteine testing experimental, investigational, or unproven for all other indications, including the following (not an all-inclusive list) because its effectiveness for these indications has not been established:

   • As a biomarker for the development and/or progression of erectile dysfunction
   • As a biomarker for risk of gastric cancer
   • As a biomarker for risk of pre-eclampsia
   • As a biomarker for carotid artery stenosis with coronary artery disease
   • As a biomarker diabetic nephropathy and diabetic retinopathy
   • As a biomarker for hematological complications of COVID-19 infection
   • As a diagnostic marker for age-related macular degeneration
   • As a diagnostic marker for Alzheimer's disease
   • Assessment of acquired thrombophilia
   • Assessment of arterial thrombosis in newborns
   • Assessment of autism (see CPB 0648 - Autism Spectrum Disorders)
   • Assessment of cardiovascular disease or stroke risk (see CPB 0381 - Cardiovascular Disease Risk Tests)
   • Assessment of cognitive impairment and dementia (e.g., Alzheimer's disease, Binswanger's disease)
   • Assessment of depression
   • Assessment of Down's syndrome
   • Assessment of fracture risk (see CPB 0562 - Biochemical Markers of Bone Remodeling)
   • Assessment of Gaucher's disease
   • Assessment of HELLP syndrome
   • Assessment of lung transplant candidates
   • Assessment of Meniere's disease
   • Assessment of migraine headaches
   • Assessment of movement disorders (e.g., Huntington's disease, Parkinson's disease, and primary dystonia) 
   • Assessment of multiple sclerosis
   • Assessment of polycystic ovary syndrome
   • Assessment of premature ovarian failure
   • Assessment of primary carnitine deficiency
   • Assessment of recurrent pregnancy loss (see CPB 0348 - Recurrent Pregnancy Loss)
   • Assessment of retinal artery occlusion
   • Assessment of retinal branch vein occlusion
   • Assessment of stroke risk
   • In-vitro fertilization planning (assessment and treatment of implantation failure)
   • Management of 5,10-methylenetetrahydrofolate reductase (MTHFR) abnormalities
   • Management of celiac disease
   • Management of inflammatory bowel disease
   • Management of pregnant members with mildly elevated anticardiolipin IgM
   • Management of pulmonary hypertension
   • Monitoring of methotrexate therapy
   • Monitoring of individuals with riboflavin transporter deficiency type-2 (RTD-2)
   • Monitoring of s-adenosylmethionine therapy
   • Monitoring response to vitamin B-12 therapy
   • Prediction of suicidal behaviors in individuals with depression
   • Prediction/diagnosis of post-stroke depression.

3. Related Policies

   • CPB 0348 - Recurrent Pregnancy Loss
   • CPB 0381 - Cardiovascular Disease Risk Tests
   • CPB 0536 - Vitamin B-12 Therapy
   • CPB 0562 - Biochemical Markers of Bone Remodeling
   • CPB 0648 - Autism Spectrum Disorders

Background

Homocysteine is an amino acid used to make protein and to build and maintain tissue. Excess levels in the blood are purported to increase the risk of stroke, certain types of heart disease or peripheral artery disease (PAD).

The most common form of genetic hyper-homocysteinemia results from production of a thermo-labile variant of methylene tetrahydrofolate reductase (MTHFR) with reduced enzymatic activity.  The gene encoding for this variant contains an alanine-to-valine substitution at amino acid 677 (C677T).  The responsible gene is common, with a population frequency estimated between 5 % to 14 %.  Homozygosity for the thermo-labile variant of MTHFR (TT genotype) is a relatively common cause of mildly elevated pHcy in the general population, often occurring in association with low serum folate levels (Rosenson and Kang, 2007).  Harmon and colleagues (1996) quantified the contribution of the thermo-labile mutation to the hyper-homocysteinemic phenotype in a working male population (n = 625).  Serum folate and vitamin B12 concentrations were measured and their relationship with Hcy status and MTHFR genotype were assessed.  They found that 11.5 % of the subjects were homozygous for the TT genotype.  However, for those in the top 5 to 10 % of pHcy, the frequency rose to 48 % and 36 %, respectively.  Homozygotes also had the lowest serum folate concentrations.

However, the role of screening for MTHFR variants during pregnancy to ascertain risks of neural tube defects (NTDs) and/or recurrent pregnancy loss is unclear.

Role of Hcy in 5,10-MTHFR Abnormalities

Finnell and associates (2002) stated that despite the fact that NTDs are the most common congenital malformations of the central nervous system, investigators have yet to identify responsible gene(s).  Research efforts have been productive in the identification of environmental factors, such as peri-conceptional folic acid supplementation that modulate risk for the development of NTDs.  Studies of the folic acid biosynthetic pathway led to the discovery of an association between elevated levels of Hcy and NTD risk.  Researchers subsequently identified single nucleotide polymorphisms in the gene coding for the enzyme 5,10-MTHFR.  Association studies suggested it was a potential risk factor for NTDs, because the thermo-labile form of the enzyme led to elevated pHcy when folic acid intake is low.  Numerous studies analyzing MTHFR variants have resulted in positive associations with increased NTD risk only in certain populations, suggesting that these variants are not large contributors to the etiology of NTDs.  With limited understanding of the genes involved in regulating NTD susceptibility, the paucity of data on how folic acid protects the developing embryo as well as the observed decrease in birth prevalence of NTDs following folic acid supplementation and food fortification, it makes little sense for prospective parents to be tested for MTHFR variants, or for variants of other known folate pathway genes.

Makino and co-workers (2004) studied whether polymorphisms of MTHFR and the endothelial nitric oxide synthase (eNOS) are associated with recurrent pregnancy loss (RPL).  They concluded that the nitric oxide concentration but not the polymorphism of MTHFR and eNOS gene and hyper-homocysteinemia are associated with RPL.

O'Leary and colleagues (2005) stated that methionine synthase reductase (MTRR) regenerates methylated cobalamin levels from the oxidized cob(II)alamin form and in so doing plays a crucial role in maintaining the active state of methionine synthase (MTR), which is an essential enzyme catalyzing the conversion of Hcy to methionine.  Single nucleotide polymorphisms (SNPs) within the MTRR gene may potentially compromise MTR activity leading to elevated pHcy, a known risk factor for NTDs.  These researchers studied the MTRR polymorphisms I22M (66A-->G), S175L (524C-->T), and K350R (1049A-->G) as potential NTD risk factors in a large homogeneous Irish NTD population.  Degree of risk was assessed via case/control comparison, log-linear analysis, and transmission disequilibrium testing.  No association was found between NTDs and I22M in mothers (p = 0.16, odds ratio [OR] 1.14 [0.95 to 1.38], n = 447) or cases (p = 0.13, OR 1.15 [0.96 to 1.38], n = 470) compared to controls (n = 476).  A dominant I22M paternal effect was found through case/control comparison and log-linear modeling (p = 0.019) (goodness-of-fit, p = 0.91, OR 1.46 [1.10 to 1.93], n = 423).  No significant NTD association was found with S175L or K350R in cases or their parents and no interactions were observed between these polymorphisms and the D919G variant of MTR or the A222V variant of 5,10-MTHFR.  These investigators also compared the frequencies of I22M, S175L, and K350R in African-Americans versus American-Caucasians.  The frequencies of I22M and K350R differed significantly between the 2 groups (p = 0.0005 and p = 0.0001, respectively).  These findings do not support an important role for these MTRR variants in NTDs.

Role of Hcy in Coronary Heart Disease or Stroke Risk

While Hcy has been reported to exhibit atherogenic and prothrombotic properties, and  histopathological hallmarks of Hcy-induced vascular injury include intimal thickening, elastic lamina disruption, smooth muscle hypertrophy, marked platelet accumulation, and the formation of platelet-enriched occlusive thrombi, its role in coronary heart disease and stroke is unclear.  In randomized trials, reduction in Hcy levels has failed to lower overall risk for cardiovascular disease (CVD).

Genest and colleagues (2000) noted that the epidemiological evidence linking total plasma Hcy to atherosclerosis is mainly derived from case-control studies, however the strength of this association is weak in prospective studies.  Thus, the causal relationship between total plasma Hcy and heart disease is not as strong as one would like to make recommendations regarding screening and treatment for the prevention of CVD.

In a double-blind, randomized controlled trial, Toole and associates (2004) examined if high doses of folic acid, vitamin B6, and vitamin B12, given to lower total Hcy levels would reduce the risk of recurrent stroke over a 2-year period compared with low doses of these vitamins.  A total of 3,680 adults with non-disabling cerebral infarction were included in this study.  Subjects received best medical and surgical care plus a daily multi-vitamin containing the United States Food and Drug Administration's reference daily intakes of other vitamins; patients were randomly assigned to receive once-daily doses of the high-dose formulation (n = 1,827), containing 25 mg of vitamin B6, 0.4 mg of vitamin B12, and 2.5 mg of folic acid; or the low-dose formulation (n = 1,853), containing 200 microg of vitamin B6, 6 microg of vitamin B12 and 20 microg of folic acid.  Main outcome measures were recurrent cerebral infarction (primary outcome); coronary heart disease (CHD) events and death (secondary outcomes).  Mean reduction of total Hcy was 2 micromol/L greater in the high-dose group than in the low-dose group, but there was no treatment effect on any end point.  The unadjusted risk ratio for any stroke, CHD event, or death was 1.0 (95 % confidence interval [CI]: 0.8 to 1.1), with chances of an event within 2 years of 18.0 % in the high-dose group and 18.6 % in the low-dose group.  The risk of ischemic stroke within 2 years was 9.2 % for the high-dose and 8.8 % for the low-dose groups (risk ratio, 1.0; 95 % CI: 0.8 to 1.3) (p = 0.80 by log-rank test of the primary hypothesis of difference in ischemic stroke between treatment groups).  There was a persistent and graded association between baseline total Hcy level and outcomes.  A 3-micromol/L lower total Hcy level was associated with a 10 % lower risk of stroke (p = 0.05), a 26 % lower risk of CHD events (p < 0.001), and a 16 % lower risk of death (p = 0.001) in the low-dose group and a non-significantly lower risk in the high-dose group by 2 % for stroke, 7 % for CHD events, and 7 % for death.  The authors concluded that in this trial, moderate reduction of total Hcy after non-disabling cerebral infarction had no effect on vascular outcomes during the 2 years of follow-up.  However, the consistent findings of an association of total Hcy with vascular risk suggested that further exploration of the hypothesis is warranted and longer trials in different populations with elevated total Hcy may be necessary.

Lewis et al (2005) stated that despite the statistical association of Hcy with vascular events, a causal association is unproven, and there is no convincing biological mechanism by which small increases in pHcy would promote CVD.  Hankey (2006) noted that there is insufficient evidence to confirm that Hcy is a modifiable causal risk factor for stroke, or to recommend routine screening for, or treatment of, raised plasma total Hcy levels with folic acid and other vitamins, to prevent ischemic stroke.  Moreover, the Thrombosis Interest Group of Canada (Houston et al, 2006) stated that there is no evidence to support routine measurement of Hcy in patients with arterial or venous disease.  The Group also noted that fasting plasma or serum Hcy concentrations may be measured as a part of the investigation of selected patients with venous thrombo-embolism, especially those with idiopathic thrombosis, recurrent thrombosis, and thrombosis at a young age or at an unusual site.  The finding of elevated Hcy by itself would not influence management, as it is a relatively weak risk factor for thrombosis and has not been shown to increase the risk of recurrence.  The rationale for its measurement is that the risk associated with elevated Hcy augments the risk associated with other thrombophilic disorders such as Factor V Leiden, and a longer duration of anti-coagulation may be warranted in patients with multiple thrombophilias.

The B-Vitamin Treatment Trialists' Collaboration (2006) reviewed the design and statistical power of 12 randomized trials assessing the effects of lowering Hcy with vitamin B supplements on risk of CVD.  The authors concluded that the strength of association of Hcy with risk of CVD may be weaker than had previously been believed.  Extending the duration of treatment in these trials would allow any effects associated with prolonged differences in Hcy concentrations to emerge.  Establishing a prospective meta-analysis of the ongoing trials of Hcy lowering should ensure that reliable information emerges about the effects of such interventions on CVD outcomes.

Lonn (2007) summarized observational studies linking Hcy to ischemic heart disease, stroke, and venous thrombo-embolism.  These studies support weak associations between Hcy and vascular risk.  A number of recent large randomized controlled studies failed to demonstrate benefit for Hcy lowering with B vitamin supplements in the prevention of cardiovascular events and venous thrombosis.  However, these trials may have been insufficiently powered to detect modest but clinically important treatment benefits.  Thus, completion of ongoing large randomized studies is essential.  The author concluded that the status of Hcy as a target for intervention in the prevention of atherothrombotic arterial and venous disease is uncertain.  Current evidence does not support the use of B vitamin supplements to reduce vascular risk.  Ongoing large randomized studies will provide further clarity on this subject.

Lazzerini et al (2007) examined the relationship between Hcy and CVD in patients affected with autoimmune diseases (ADs), reviewing the most recent literature data and also providing their experience.  Although the large amount of available studies showed that mild hyper-homocysteinemia represents a common finding in patients affected with several autoimmune diseases, the actual role of Hcy in the development of CVD in the course of AD is still unclear, perhaps, with the only exception of the systemic lupus erythematosus.  In the other conditions, the role of Hcy in the pathogenesis of vascular complications is still a matter of debate, as the result of conflicting reports and/or lack of an adequate body of investigation.

In a double-blind, randomized controlled trial, Jamison et al (2007) examined if high doses of folic acid and B vitamins administered daily would reduce mortality in patients with chronic kidney disease (CKD).  Median follow-up was 3.2 years for 2,056 participants aged 21 years or older with advanced CKD (estimated creatinine clearance less than or equal to 30 mL/min) (n = 1,305) or end-stage renal disease (n = 751) and high Hcy levels (greater than or equal to 15 micromol/L).  Subjects received a daily capsule containing 40 mg of folic acid, 100 mg of vitamin B6, and 2 mg of vitamin B12 or a placebo.  The primary outcome was all-cause mortality.  Secondary outcomes included myocardial infarction (MI), stroke, amputation of all or part of a lower extremity, a composite of these 3 plus all-cause mortality, time to initiation of dialysis, and time to thrombosis of arterio-venous access in hemodialysis patients.  Mean baseline Hcy level was 24.0 micromol/L in the vitamin group and 24.2 micromol/L in the placebo group.  It was lowered 6.3 micromol/L (25.8 %; p < 0.001) in the vitamin group and 0.4 micromol/L (1.7 %; p = 0.14) in the placebo group at 3 months, but there was no significant effect on mortality (448 vitamin group deaths versus 436 placebo group deaths) (hazard ratio [HR], 1.04; 95 % CI: 0.91 to 1.18).  No significant effects were demonstrated for secondary outcomes or adverse events: there were 129 MIs in the vitamin group versus 150 for placebo (HR, 0.86; 95 % CI: 0.67 to 1.08), 37 strokes in the vitamin group versus 41 for placebo (HR, 0.90; 95 % CI: 0.58 to 1.40), and 60 amputations in the vitamin group versus 53 for placebo (HR, 1.14; 95 % CI: 0.79 to 1.64).  In addition, the composite of MI, stroke, and amputations plus mortality (p = 0.85), time to dialysis (p = 0.38), and time to thrombosis in hemodialysis patients (p = 0.97) did not differ between the vitamin and placebo groups.  The authors concluded that treatment with high doses of folic acid and B vitamins did not improve survival or reduce the incidence of vascular disease in patients with advanced CKD or end-stage renal disease.

In a randomized, double-blind, placebo-controlled study, Albert and colleagues (2008) examined if a combination of folic acid, vitamin B6, and vitamin B12 would lower risk of CVD among high-risk women with and without CVD.  A total of 5,442 women who were United States health professionals aged 42 years or older, with either a history of CVD or 3 or more coronary risk factors, received a combination pill containing folic acid (2.5 mg), vitamin B6 (50 mg), and vitamin B12 (1 mg) or a matching placebo, and were treated for 7.3 years.  Main outcome measures were a composite outcome of myocardial infarction, stroke, coronary re-vascularization, or CVD mortality.  Compared with placebo, a total of 796 women experienced a confirmed CVD event (406 in the active group and 390 in the placebo group).  Patients receiving active vitamin treatment had similar risk for the composite CVD primary end point (226.9/10,000 person-years versus 219.2/10,000 person-years for the active versus placebo group; relative risk [RR], 1.03; 95 % CI: 0.90 to 1.19; p = 0.65), as well as for the secondary outcomes including myocardial infarction (34.5/10,000 person-years versus 39.5/10,000 person-years; RR, 0.87; 95 % CI: 0.63 to 1.22; p = 0.42), stroke (41.9/10,000 person-years versus 36.8/10,000 person-years; RR, 1.14; 95 % CI: 0.82 to 1.57; p = 0.44), and CVD mortality (50.3/10,000 person-years versus 49.6/10,000 person-years; RR, 1.01; 95 % CI: 0.76 to 1.35; p = 0.93).  In a blood substudy, geometric mean pHcy was decreased by 18.5 % (95 % CI: 12.5 % to 24.1 %; p < 0.001) in the active group (n = 150) over that observed in the placebo group (n = 150), for a difference of 2.27 micromol/L (95 % CI: 1.54 to 2.96 micromol/L).  The authors concluded that after 7.3 years of treatment and follow-up, a combination pill of folic acid, vitamin B6, and vitamin B12 did not reduce a combined end point of total cardiovascular events among high-risk women, despite significant Hcy lowering.

In an editorial that accompanied the afore-mentioned article, Lonn (2008) stated that currently vitamin B supplements can not be recommended for the prevention of CVD events with the exception of rare genetic disorders, and there is no role for routine screening for elevated Hcy levels.

A Cochrane systematic evidence review (Martí-Carvajal et al, 2009) concluded that "[r]esults from available published trials suggest that there is no evidence to support the use of HLI [homocysteine lowering interventions] to prevent cardiovascular events."

The U.S. Preventive Services Task Force (USPSTF, 2009) concluded that the evidence is insufficient to assess the balance of benefits and harms of using Hcy levels to screen asymptomatic men and women with no history of CHD to prevent CHD events.

The American Academy of Family Physicians' "Summary of recommendations for clinical preventive services" (AAFP, 2012) concludes that for coronary heart disease (CHD), the current evidence is insufficient to evaluate the balance of benefits and harms of using the non-traditional risk factors discussed in this statement to screen asymptomatic men and women with no history of CHD to prevent CHD events.  The non-traditional risk factors included in this recommendation are high-sensitivity C-reactive protein, ankle-brachial index, leukocyte count, fasting blood glucose level, periodontal disease, carotid intima-media thickness, coronary artery calcification score on electron-beam computed tomography, Hcy level, and lipoprotein(a) level.

In a post-hoc analysis of the China Stroke Primary Prevention Trial (CSPPT), Huang and colleagues (2017) examined if a change in serum tHcy levels is associated with first stroke risk.  These researchers analyzed 16,867 participants of the CSPPT with tHcy measurements at both baseline and exit visits.  The primary outcome was first stroke; the secondary outcome was a composite of CV events consisting of CV death, MI, and stroke.  The percent decline in tHcy was calculated as [(baseline tHcy - exit tHcy)/baseline tHcy × 100].  Over the median treatment duration of 4.5 years, participants who developed a 1st stroke had a significantly lower percent decline in tHcy (β = -5.7; 95 % CI: -8.8 to -2.6) compared to their counterparts.  A 20 % tHcy decline was associated with a reduction in stroke risk of 7 % (HR 0.93; 95 % CI: 0.90 to 0.97).  When percent decline in tHcy was assessed as tertiles, a significantly lower stroke risk was found in those in tertiles 2-3 (HR 0.79; 95 % CI: 0.64 to 0.97) compared with participants in tertile 1.  Similar results were observed for the composite of CV events.  The beneficial effect associated with greater tHcy reduction was observed across strata for age, sex, treatment group (with versus without folic acid), MTHFR C677T genotypes, baseline tHcy and serum folate levels, and blood pressure (BP) control.  The authors concluded that percent lowering in tHcy was significantly associated with a reduction in 1st stroke risk in Chinese adults with hypertension, and if further confirmed, may serve as a useful indicator for folic acid treatment efficacy on stroke prevention. 

The authors stated that this study had several drawbacks.  First, post-hoc analyses of randomized trials have inherent limitations, such as the possibility of residual imbalance in some unmeasured predictive factors at baseline.  Second, these investigators used a fixed dose of folic acid (0.8 mg daily) alone without including other B vitamins.  However, a previous study reported that vitamin B12 (median 0.4 mg/day) only produced a 7 % (95 % CI: 4 % to 9 %) further reduction in tHcy, and vitamin B6 had no significant effect.  Moreover, it has been suggested that the beneficial effect of folic acid therapy could possibly be offset by the toxicity associated with a high-dose of cyanocobalamin, especially among patients with impaired renal function.  Third, tHcy was assessed only at the baseline and the exit visits.  More frequent measurements of tHcy would allow for a more accurate assessment of the association between the change in tHcy and stroke risk.  Thus these researchers stated that confirmation of these findings in more studies is essential.

Ghose et al (2023) noted that stroke is a major leading global health complication.  Identification and management of risk factors associated with stroke can help in prior detection, prevention, and improvement in patient care.  These investigators examined the prevalence of hyperhomocysteinemia (HHcy) and vitamins B6, B12, and folate deficiency in stroke patients and evaluated other risk factors associated with ischemic and hemorrhagic stroke.  Detail history of all the subjects in the study including history of hypertension, anemia, fasting glucose, carotid artery thickness, smoking, alcohol, and dietary intake was recorded.  Standard assays for Hcy, vitamins B6, B12, and folate estimation were carried out; lipid and renal profile tests were also performed.  The prevalence and odds of having HHcy, vitamins B6, B12, and folate deficiency, and other risk factors in ischemic and hemorrhagic stroke patients were evaluated.  Student's t-tests and Chi-square tests were carried out for statistical validation of the data.  Prevalence of HHcy and vitamins B6, B12, and folate deficiency was not observed in ischemic cases.  HHcy and folate deficiency was found to be prevalent in hemorrhagic stroke patients.  The odds that an individual with HHcy and folate deficiency has hemorrhagic stroke was found to be significantly high.  The authors concluded that high Hcy and low folate levels emerged as risk factors for hemorrhagic stroke.  These investigators stated that the findings of this study support the hypothesis that HHcy and folate deficiency was associated with hemorrhagic stroke; however, a similar conclusion could not be drawn for ischemic stroke.

The authors stated that the main drawback of this study was the relatively small size of cases and controls (n = 110 in each group).  For the estimation of normal levels of tHcy among the general population, a large number of controls need to be considered.  Furthermore, these researchers were unable to measure tHcy levels in different subgroups.  They could not study the correlation of the levels with the stroke outcome; thus, more large-scale studies are needed for further verification.

Role of Hcy in Gastric Cancer Risk

Wang and colleagues (2020) noted that the association of plasma homocysteine levels (PHL) with gastric cancer risk was reported in observational studies.  However, the causality is challenging due to confounding factors and the lack of evidence from well-designed cohort studies.  These researchers carried out a 2-sample Mendelian randomization (MR) analysis to examine if PHL is causally related to gastric cancer risk.  They conducted the MR analysis based on the results from genome-wide association studies consisting of 2,631 patients with gastric cancer and 4,373 controls.  An externally weighted genetic risk score (wGRS) was constructed with 15 SNPs with well-established associations with PHL.  These investigators employed logistic regression model to estimate associations of PHL-related SNPs and wGRS with gastric cancer risk in total population and in strata by sex, age, and study site, in addition to a series of sensitivity analyses.  High genetically predicted PHL was associated with an increased gastric cancer risk (per SD increase in the wGRS: OR = 1.07; 95 % CI: 1.01 to 1.12; p = 0.011), which was consistent in sensitivity analyses. Subgroup analyses provided evidence of a stronger association with gastric cancer risk in women than in men. MR-Egger and weighted median regression suggested that potentially unknown pleiotropic effects were not biasing the association between PHL and gastric cancer risk.  The authors concluded that these findings demonstrated that genetically predicted high PHL was associated with an increased gastric cancer risk, suggesting that high PHL may have a causal role in the etiology of gastric cancer.  These preliminary findings need to be validated by well-designed studies.

Role of Hcy in Multiple Sclerosis

Vrethem and colleagues (2003) examined if multiple sclerosis (MS) is associated with vitamin B12 deficiency.  These researchers measured serum vitamin B12, plasma folate, serum methylmalonic acid (MMA), pHcy as well as cerebrospinal fluid (CSF) MMA and Hcy in 72 patients with MS and 23 controls.  The mean pHcy level was significantly increased in MS patients (11.6 micromol/L) compared with controls (7.4 micromol/L) (p = 0.002).  Seven patients showed low serum vitamin B12 levels but only 1 of them had concomitant high pHcy.  None of them showed high serum MMA.  Plasma or blood folate levels did not differ between MS patients and controls.  These researchers found no significant differences in mean values or frequency of pathological tests of serum B12, serum MMA, mean corpuscular volume (MCV), hemoglobin concentration, CSF Hcy or CSF MMA between patients and healthy subjects.  There were no correlations between CSF and serum/plasma levels of MMA or Hcy.  Serum vitamin B12, serum MMA, pHcy, CSF Hcy or CSF MMA were not correlated to disability status, activity of disease, duration of disease or age.  The authors concluded that the relevance of the increased mean value of pHcy thus seems uncertain and does not indicate functional vitamin B12 deficiency.  However, they can not exclude the possibility of a genetically induced dysfunction of the Hcy metabolism relevant for the development of neuro-inflammation/degeneration.  These findings indicated that, regardless of a significant increase in pHcy in MS patients, the disease is not generally associated with vitamin B12 deficiency since they did not find any other factors indicating vitamin B12 deficiency.  Analysis of CSF MMA and CSF Hcy, which probably reflects the brain vitamin B12 status better than serum, are not warranted in MS.  The authors concluded that B12 deficiency, in general, is not associated with MS.

Ramsaransing and associates (2006) stated that there is evidence that Hcy contributes to various neurodegenerative disorders, and elevated pHcy levels have been observed in patients with MS.  These investigators examined if and why pHcy levels are increased in MS, and whether they play a role in the disease course.  They compared pHcy in 88 patients with MS and 57 healthy controls.  In the MS group, 28 had a benign course, 37 were secondary progressive, and 23 primary progressive.  To explore the underlying mechanisms, these investigators measured serum levels of vitamins B6 and B12, folate, interleukin (IL)-12, tumor necrosis factor (TNF)-alpha, leukocyte nitric oxide production, and plasma diene conjugate levels (measure of oxidative stress).  Mean pHcy was higher in patients (13.8 micromol/L) than in controls (10.1 micromol/L; p < 0.0001).  However, there were no significant differences in Hcy levels between the three clinical subgroups of MS.  Serum concentrations of vitamin B6, vitamin B12, and folate were not different between patients with MS and controls.  In the MS group, there were no correlations between pHcy and the serum levels of IL-12 or TNF-alpha, leukocyte nitric oxide production, or plasma diene conjugate levels.  The authors concluded that elevated pHcy occurs in both benign and progressive disease courses of MS, and seems unrelated to immune activation, oxidative stress, or a deficiency in vitamin B6, vitamin B12, or folate.

Role of Hcy in Polycystic Ovary Syndrome

Badawy and colleagues (2007) examined the relationship between insulin resistance and increased serum Hcy in women with polycystic ovarian syndrome (PCOS).  A total of 90 PCOS women as a study group and 35 women with infertility due to other causes as a control group were enrolled in this study.  Outcome measures included serum Hcy levels in the presence and absence of insulin resistance in PCOS patients.  Homocysteine levels were significantly higher in PCOS patients than in the controls.  Considering 11 micromol/l as the cut-off level for a normal Hcy level, 41.1 % of PCOS patients (37 out of 90) and 2.9 % of control group (1 out of 35) had high Hcy levels.  With regard to insulin resistance, 23 % of PCOS patients without insulin resistance (9 out of 39) had a high Hcy level, while 47 % of PCOS patients with insulin resistance (24 out of 51) had high Hcy level, thus demonstrating the effect of insulin resistance on the Hcy level.  The authors concluded that there is a strong association between serum Hcy and insulin resistance in women with PCOS that contributes to the long-term complications of PCOS.
 
On the other hand, Schachter et al (2007) reported that in women with insulin-resistant PCOS, pHcy were significantly reduced by both B vitamins and metformin, but to a greater degree by B vitamins, and higher pregnancy rates were associated with vitamin B treatment.  Also, Carlsen et al (2007) reported that metformin treatment in women with PCOS does not increase serum Hcy levels in the non-pregnant or the pregnant state.  There is currently a lack of evidence regarding the association of monitoring of Hcy levels in women with PCOS and "fertility".  Furthermore, Battaglia et al (2008) noted that PCOS is a condition associated with an increased vascular risk, however, the use of Hcy testing for assessing the risk of CHD has not been established.

Role of Hcy in Osteoporosis / Fractures

High Hcy levels in adults have been associated with osteoporotic fractures in some, but not all, studies.  However, it is unclear if high levels of Hcy have a direct effect on bone or if the effect is mediated through another factor, such as poor nutrition (Rosenson and Kang, 2007).

In a double-blind, randomized controlled trial, Sato et al (2005) examined if treatment with folate and vitamin B12 would reduce the incidence of hip fractures in patients with hemiplegia following stroke.  A total of of 628 consecutive patients aged 65 years or older with residual hemiplegia at least 1 year following first ischemic stroke were included in this study.  Patients were assigned to daily oral treatment with 5 mg of folate and 1,500 microg of vitamin B12, or double placebo; 559 completed the 2-year follow-up.  Main outcome measure was incidence of hip fractures in the 2 patient groups during the 2-year follow-up.  At baseline, patients in both groups had high levels of plasma Hcy and low levels of serum cobalamin and serum folate.  After 2 years, pHcy decreased by 38 % in the treatment group and increased by 31 % in the placebo group (p < 0.001).  The number of hip fractures per 1000 patient-years was 10 and 43 for the treatment and placebo groups, respectively (p < 0.001).  The adjusted relative risk, absolute risk reduction, and the number needed to treat for hip fractures in the treatment versus placebo groups were 0.20 (95 % CI: 0.08 to 0.50), 7.1 % (95 % CI: 3.6 % to 10.8 %), and 14 (95 % CI: 9 to 28), respectively.  No significant adverse effects were reported.  The authors concluded that in this Japanese population with a high baseline fracture risk, combined treatment with folate and vitamin B12 is safe and effective in reducing the risk of a hip fracture in elderly patients following stroke.  An editorial that accompanied this article (van Meurs and Uitterlinden, 2005) noted that the final proof of causality between circulating Hcy levels and fracture risk will have to come from elucidation of the biological mechanism underlying this relationship.

Selhub (2006) noted that elevated pHcy is associated with increased total and CVD mortality, increased incidence of stroke, increased incidence of dementia and Alzheimer's disease, increased incidence of bone fracture, and higher prevalence of chronic heart failure.  This multitude of relationships between elevated plasma total Hcy (tHcy) and diseases that afflict the elderly point to the existence of a common denominator that may be responsible for these diseases.  The author stated that whether this denominator is Hcy itself or whether Hcy is merely a marker remains to be determined.

In a population-based prospective study, Gjesdal et al (2007) examined if plasma levels of tHcy, folate, and vitamin B12 and the MTHFR 677C-->T and 1298C-->T polymorphisms predicted hip fracture.  A total of 2,639 women and 2,127 men who were 65 to 67 years of age were included in this study.  Cox proportional hazard regression was used to estimate fracture risk according to levels of plasma tHcy, folate, and vitamin B12 and for different genotypes.  Over a median follow-up period of 12.6 years, hip fracture was recorded in 184 (7.0 %) women and 90 (4.2 %) men.  The adjusted hazard ratio (95 % CI) for fracture in subjects with high (greater than or equal to 15 micromol) compared with low levels (less than 9.0 micromol) of tHcy was 2.42 (1.43 to 4.09) among women and 1.37 (0.63 to 2.98) among men.  Dose-response analyses indicated a positive association between plasma tHcy and risk of fracture in both sexes and a negative association between plasma folate and risk of fracture among women only.  Plasma vitamin B12 level or MTHFR genotype was not significantly related to risk of fracture after adjustments for confounding factors.  The association between tHcy and risk of hip fracture was only slightly weakened by adjustments for plasma levels of vitamin B12 and folate.  The authors concluded that tHcy seems to be a predictor for hip fracture among elderly men and women.  Folate was a predictor among women only, whereas vitamin B12 and MTHFR genotype did not predict hip fracture.  These findings corroborated the hypothesis that Hcy may play a role in the pathogenesis of osteoporotic fractures.

Role of Hcy in Recurrent Pregnancy Loss

Hague (2003) stated hyper-homocysteinemia has been associated with vascular disease, although whether it is cause or effect is still a matter of debate.  In normal pregnancy, Hcy concentrations fall.  Disturbance of maternal and fetal Hcy metabolism has been associated with fetal NTDs, with various conditions characterized by placental vasculopathy, such as pre-eclampsia and abruption, and with recurrent pregnancy loss.  Apart from folate supplementation, which has been clearly shown to halve the risk of fetal NTDs, no other strategies have been identified in relation to Hcy metabolism that will reliably reduce the frequency of these other common obstetric pathologies.

ACOG guidelines on inherited thrombophilias in pregnancy (2014) state that, because of the lack of association between heterozygosity or homozygosity for MTHFR C677T polymorphism and any negative pregnancy outcomes, including any increased risk for venous thrombo-embolsim, screening with either MTHFR mutation analyses or fasting homocysteine levels is not recommended. 

Role of Hcy in Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency

Complications of homocystinuria should be treated appropriately (e.g., surgical intervention for ectopia lentis).  Treatments should aim to correct the biochemical abnormalities, especially to control pHcy and prevent thrombosis.  Individuals identified by newborn screening are treated shortly after birth to maintain pHcy below 11 micromol/L.  For newborn screening, measurements of Hcy (plasma and/or urine) are performed only when hypermethioninemia has been confirmed.  Measurement of plasma concentrations of amino acids and Hcy in at-risk siblings immediately after birth ensures reduction of morbidity and mortality by early diagnosis and treatment.  Prophylactic anti-coagulation during the 3rd trimester of pregnancy and post-partum in women with homocystinuria is recommended to reduce risk of thrombo-embolism (Picker and Levy, 2006).

Role of Hcy in In-Vitro Fertilization Planning

The role of Hcy testing in in-vitro fertilization (IVF) or intra-cytoplasmic sperm injection (ICSI) has not been established.  Jerzak and colleagues (2003) determined fasting tHcy in follicular fluid or serum of women experiencing reproductive failure after spontaneous or IVF.  A total of 8 non-pregnant recurrent spontaneous abortion (RSA) women, 8 normal healthy women with previous successful pregnancy outcome and 15 women undergoing IVF (6 with unexplained infertility [UI], 6 with male factor [MF], 3 with tubal obstruction [TO]) were enrolled in this study.  Total fasting Hcy concentrations were established by ELISA method in serum of RSA and normal healthy women or ovarian follicular fluid of women undergoing IVF.  Mean fasting tHcy concentration in study group were as follows: RSA: 18.63 mumol/L +/- 6.67, normal: 13.98 mumol/L +/- 6.62, UI: 20.62 mumol/L +/- 8.19, MF: 22.60 mumol/L +/- 7.87, TO: 36.75 mumol/L +/- 13.26.  These researchers found that RSA women have had significantly higher serum Hcy concentration when compared to normal healthy women (p < 0.05).  Among women undergoing IVF, those with following IVF success had not significantly lower Hcy level when compared to those with IVF failure (22.81 mumol/L +/- 11.27 versus 24.54 mumol/L +/- 9.50, p > 0.05).  The authors concluded that these preliminary data suggested that high hcy level may negatively influence pregnancy outcome following natural or IVF.  It can not be excluded that elevated Hcy concentrations contribute to defective chorionic villous vascularization during early stages of gestation.

Pacchiarotti and associates (2007) evaluated the effect of the hyperhomocysteinemia on pregnancy rate, implantation rate and abortion rate after IVF.  Data from a total of 48 infertile couples with hyperhomocysteinemia were prospectively collected for this study.  All patients underwent a standard down regulation protocol for ovarian stimulation.  Oocytes recovery was performed at 36 hrs after hCG administration.  Embryo transfer took place at 48 hrs after insemination.  The patients were matched in 2 groups that received or did not receive therapy (group A and group B, respectively) to normalize Hcy plasma level.  Pregnancy rate, implantation rate and abortion rate varied significantly (p < or = 0.05) between the 2 groups.  The number and quality of embryos transferred did not differ between the groups.  The authors concluded that the results suggested that hyper-homocysteinemia could affect IVF outcome.

Boxmeer et al (2009) examined if biomarkers of the Hcy pathway are associated with IVF outcome.  These researchers investigated biomarkers of the Hcy pathway for associations with embryo quality and biochemical pregnancy in women undergoing IVF or ICSI treatment (n = 181).  In the treatment cycle, blood and mono-follicular fluid samples were collected for determination of folate, cobalamin and tHcy concentrations.  Of all the women in the study, 67 % used folic acid supplements.  In blood, a significant correlation was established between high cobalamin and better embryo quality [standardized adjusted regression coefficient: -0.17, 95 % CI: -0.30 to -0.01].  In mono-follicular fluid of non-supplemented women, high cobalamin correlated with better embryo quality (estimate: -0.87; 95 % CI: -1.68 to -0.06), whereas high tHcy resulted in poor embryo quality (estimate: 1.01; 95 % CI: 0.08 to 1.95).  However, in mono-follicular fluid of supplemented women, high tHcy correlated with better embryo quality (estimate: -0.58; 95 % CI: -1.12 to -0.04).  In the total group, a 2-fold increase of monofollicular fluid folate corresponded with a 3.3 times higher chance (95 % CI: 1.09 to 9.71) of achieving pregnancy.  The authors concluded that an optimal Hcy pathway in follicular fluid is associated with a better embryo quality and chance of pregnancy.

Nafiye and colleagues (2010) examined the serum and follicular fluid concentrations of insulin resistance parameters and Hcy and their effect on IVF outcome in non-obese, non-hyper-androgenemic polycystic ovary syndrome (PCOS) patients.  A total of 97 women underwent IVF; subjects were categorized according to IVF indications: group 1 with PCOS, group 2 with sub-fertile male partners, group 3 with UI.  Serum and follicular fluid parameters from the first follicle on the day of oocyte retrieval were analyzed.  Serum and follicular fluid insulin resistance parameters, Hcy, sex hormone levels, and laboratory and clinical IVF outcome were studied.  Serum insulin, homeostasis model assessment estimate of insulin resistance (HOMA-IR), and Hcy levels were significantly higher in subjects having PCOS.  However, these significant differences in serum insulin resistance and Hcy levels were not seen in the follicular microenvironment.  There were no differences in clinical pregnancy rates between study groups.  The authors concluded that despite elevated serum insulin, HOMA-IR, and Hcy levels, and their effects on oocyte numbers and maturation in PCOS patients, there were no differences in follicular parameters and clinical pregnancy rates between hyper-insulinemic and hyper-homocysteinemic PCOS patients and the other 2 groups.

Role of Hcy in Retinal Artery Occlusion

Weger et al (2002) stated that hyperhomocysteinemia has been established as an important risk factor for cardiovascular diseases.  These researchers examined if hyperhomocysteinemia and/or homozygosity for the MTHFR C677T mutation are associated with an increased risk for retinal artery occlusion (RAO).  In a retrospective case-control study, these investigators studied 105 consecutive patients with RAO and 105 age- and sex-matched control subjects.  Fasting plasma Hcy levels were determined by high-performance liquid chromatography, while genotypes of the MTHFR C677T mutation were determined by polymerase chain reaction.  Mean plasma Hcy levels were significantly higher in patients with RAO compared with control subjects (12.2 +/- 4.8 micromol/L versus 10.3 +/- 3.4 micromol/L; p = 0.003).  Hyper-homocysteinemia was defined by the 95th percentile of control plasma Hcy levels as 15.8 micromol/L; 20 (19.1 %) patients with RAO exceeded this level and were therefore classified as hyperhomocysteinemic compared with 5 (4.8 %) control subjects (p = 0.003).  The OR for these patients was calculated at 4.7 (95 % CI: 1.5 to 15.1).  Mean plasma folate levels were significantly lower in patients than in the control group (5.6 +/- 2.3 ng/ml versus 6.3 +/- 2.5 ng/ml; p = 0.04).  The prevalence of the homozygous genotype of MTHFR C677T mutation did not significantly differ between patients and controls.  The authors concluded that these findings suggested that hyperhomocysteinemia, but not homozygosity, for the MTHFR C677T mutation is associated with RAO.

Hong et al (2011) evaluated the diagnostic efficacy of plasma tHcy and C-reactive protein (CRP) levels for ocular ischemic syndrome (OIS).  In all, 87 patients with retinal vein occlusion (RVO), 955 patients with a stenosis of internal carotid artery (ICA) less than 90 % and 159 patients with a stenosis of ICA greater than 90 % were included between 2003 and 2009.  A total of 43 patients with a stenosis ICA greater than 90 % were diagnosed as OIS.  Fasting tHcy, CRP, lipid profiles, creatinine were measured, and diagnostic values of hyperhomocysteinemia or elevated CRP for OIS were evaluated.  The mean plasma levels of tHcy (18.8 μmol/L) and CRP (1.1 mmol/L) were the highest in patients with OIS among the groups.  The prevalence of hyperhomocysteinemia (72 %) and elevated CRP (77 %) were the highest in OIS among the groups.  In patients with stenosis of ICA, the diagnostic sensitivity/specificity for OIS was 70/79 % in hyperhomocysteinemia and 73/73 % in elevated CRP.  The diagnostic sensitivity and specificity for OIS were 53 and 86 % in both hyperhomocysteinemia and elevated CRP.  The lipid profiles and creatinine levels were similar among the groups.  The authors concluded that these findings suggested that hyperhomocysteinemia and elevated CRP may be associated with the development of OIS.  The measurements of tHcy and CRP in blood may help to assist the diagnosis of OIS in a stenosis of ICA.

Parchand et al (2012) reported primary branch RAO in a case with idiopathic retinal vasculitis, aneurysms, and neuro-retinitis syndrome.  These researchers performed a review of medical case records, color fundus photographs, and fundus fluorescein angiography of a 23-year old man diagnosed with idiopathic retinal vasculitis, aneurysms, and neuro-retinitis.  The patient presented with sudden painless decreased vision in right eye since 1 day.  Ocular examination revealed a best-corrected visual acuity (BCVA) of counting fingers 1 feet in the right eye and 20/20 in the left eye, relative afferent pupillary defect in the right eye, 1+ vitreous cells in both eyes, optic disk neovascularization with massive peri-papillary and perivascular lipid exudation, and occluded smaller vessels in both eyes.  In addition, there was an area of retinal opacification in the posterior pole along the infero-temporal arcade in the right eye.  The patient underwent color fundus photography, fluorescein angiography, and a detailed systemic work-up.  Fundus fluorescein angiographic features were suggestive of idiopathic retinal vasculitis, aneurysms, and neuro-retinitis in both eyes with branch RAO in the right eye.  Detailed systemic work-up revealed raised serum Hcy levels.  The patient underwent scatter retinal photocoagulation in both eyes and also was started on folic acid and pyridoxine supplementation.  At 1 year of follow-up, the BCVA in the right eye improved to 20/60.  Posterior segment examination showed reduced exudation, resolution of optic disk neovascularization in both eyes, and clearing of retinal opacification in the right eye.  Also, the levels of serum Hcy decreased over 1 year.  The authors concluded that primary branch RAO can be an atypical presentation of idiopathic retinal vasculitis, aneurysms, and neuro-retinitis syndrome.  Such a case should be thoroughly investigated for underlying hyper-coagulable state.  Also, a careful long-term follow-up is needed for these patients to prevent any neovascularization sequelae.

Coban-Karatas et al (2013) described the case of a child with central RAO and hyperhomocysteinemia.  A 13-year old girl developed sudden vision loss and was hospitalized for diagnosis and treatment.  Her physical examination was normal except for her ophthalmologic examination.  Her serum Hcy level and lipoprotein(a) were elevated to 45.27 μmol/L and 61 mg/dL, respectively.  A homozygous mutation was identified for MTHFR at position C677T.  The authors concluded that this report documented central RAO was associated with the risk factors of hyperhomocysteinemia caused by MTHFR C677 T mutation and high lipoprotein(a) level in a child.  Retinal artery occlusion is rare in children.  This case emphasized the need for a systemic evaluation for hyperhomocysteinemia and lipoprotein(a) levels in children with retinal vascular occlusion of uncertain etiology.

Furthermore, an eMedicine review on “Branch retinal artery occlusion” (Nathan, 2014) as well as an UpToDate review on “Central and branch retinal artery occlusion” (Hedges, 2015) do not mention homocysteine testing as a diagnostic tool.

Riboflavin Transporter Deficiency Type-2

O'Callaghan and associates (2019) stated that riboflavin transporter deficiency (RTD) is a rare neurological disorder that entails the Brown-Vialetto-Van Laere and Fazio-Londe syndromes since the discovery of pathogenic mutations in the SLC52A2 and SLC52A3 genes that encode human riboflavin transporters RFVT2 and RFVT3.  Patients present with a deteriorating progression of peripheral and cranial neuropathy that causes muscle weakness, vision loss, deafness, sensory ataxia, and respiratory compromise which when left untreated can be fatal.  Considerable progress in the clinical and genetic diagnosis of RTDs has been made in recent years and has permitted the successful life-saving treatment of many patients with high-dose riboflavin supplementation.  These investigators outlined the importance of riboflavin and its efficient transmembrane transport in human physiology.  Reports on 109 patients with a genetically confirmed diagnosis of RTD were summarized in order to highlight commonly presenting clinical features and possible differences between patients with pathogenic SLC52A2 (RTD2) or SLC52A3 (RTD3) mutations.  These researchers also focused attention on recent work with different models of RTD that have revealed possible patho-mechanisms contributing to neurodegeneration in patients.

Amir and colleagues (2020) identified symptoms and health care interactions with patients with RTD type-2 prior to diagnosis.  Parents of children with riboflavin transporter deficiency type-2 (n = 10) were interviewed to collect data on the patient's clinical journey.  The average diagnostic delay was 27.6 months.  Neurologists were the most commonly visited clinician (90 %).  Common symptoms during the 1st year of the patient's clinical journey included abnormal gait and/or ataxia (70 %), nystagmus (50 %), and upper body muscle weakness (40 %).  Prior to diagnosis, optic atrophy, sleep apnea, breath-holding spells, and dysphagia were commonly observed.  Hearing loss was only reported in 40 % of subjects prior to diagnosis.  Riboflavin responsive megaloblastic anemia was reported for the first time.  Mitochondrial disease was the most common suspected diagnosis (30 %).  The authors concluded that despite clinical variability, common early symptoms of riboflavin transporter deficiency type-2 exist that could better allow clinicians to more rapidly identify riboflavin transporter deficiency type-2.

There is  lack of evidence on homocysteine monitoring in patients with RTD type-2.

Role of Hcy in Other Conditions

There is conflicting evidence regarding whether Hcy is an independent risk factor for dementia (e.g., Alzheimer's disease, Binswanger's disease) (Rosenson and Kang, 2007).  Silbert et al (2008) examined the association of plasma Hcy and C-reactive protein (CRP) with cognition in patients scheduled for coronary artery bypass graft (CABG) surgery.  Cognition was assessed in 264 patients using a standard battery of neuropsychological tests.  Patients were classified as having pre-existing cognitive impairment (PreCI) by reference to a healthy control group or post-operative cognitive dysfunction (POCD) by reference to baseline test scores.  PreCI was present in 37.3 % of patients, and POCD was present in 18.3, 12.1 and 13.6 % of patients at 1 week, 3 months and 12 months post-operatively.  On multi-variate analysis, neither Hcy nor CRP was independently associated with cognition at any testing time but both were strongly associated with age and left ventricular function.  The authors concluded that PreCI and POCD are present in a substantial proportion of patients undergoing CABG surgery but there is no independent association with either baseline Hcy or CRP levels.  It is possible that cognitive impairment may result from the vascular disease rather than a direct association with either Hcy or CRP.  Teper and O'Brien (2008) stated that the relationship between vascular disease and depression can not be solely explained by current established risk factors or the effects of treatment for depression.  Other mechanisms must apply, and there is some evidence for common genetic factors.  Promising future lines of investigation include Hcy, cytokines and endothelial dysfunction.  They noted that more longitudinal studies combined with measurements of these biomarkers are needed. 

Ho and colleagues (2011) stated that high Hcy has been causatively linked to Alzheimer disease (AD) and vascular dementia (VaD) in old age, but research methodologies and outcome measures are heterogeneous. It remains unclear whether the findings can be generalized across studies.  In a systematic review/meta-analysis examined if high Hcy level is a risk factor for cognitive decline in elderly.  Random-effects meta-analyses were conducted on studies examining the relationship between Hcy level and risk of developing dementia/cognitive decline between comparison groups.  Meta-regression identified patient- and trial-related factors, which may contribute to heterogeneity.  A total of 17 relevant studies (6,122 participants; 13 cross-sectional and 4 prospective studies) were included.  Compared with controls, Hcy was significantly elevated in AD (pooled standardized mean difference [SMD]: 0.59; 95 % CI: 0.38 to 0.80; significant heterogeneity: τ = 0.105) and VaD (pooled SMD: 1.30; 95 % CI: 0.75 to 1.84; significant heterogeneity: τ = 0.378).  Meta-regression identified mean age as significant moderator for AD versus controls and mean age and mean folate levels as significant moderators for VaD versus controls.  Homocysteine was significantly higher in VaD relative to AD (pooled SMD: 0.48; 95 % CI: 0.23 to 0.73; moderately significant heterogeneity: τ = 0.076); proportion of men and mean folate levels were significant moderators.  High-Hcy level was not associated with risk of developing dementia in prospective studies (pooled odds ratio: 1.34; 95 % CI: 0.94 to 1.91, non-significant heterogeneity: τ = 0.048).  The authors concluded that individuals with AD and VaD have higher Hcy levels than controls; however, a causal relationship between high-Hcy level and risk of developing dementia is not supported.  They stated that more prospective studies and randomized controlled trials are required to test the therapeutic benefits of lowering Hcy levels.

Almeida et al (2008) stated that the prevalence of depression in later life increases with plasma tHcy.  High tHcy accounts for about 15 % of prevalent cases, but observational studies are prone to confounding and bias.  Genetic association studies are not prone to the same sources of error and offer an opportunity to explore the consistency and external validity of this association.  These researchers examined if tHcy is causally related to depression in later life.  A total of 3,752 men aged 70 years or older (Health in Men Study) were included in this study.  Main outcome measure were 15-item Geriatric Depression Scale and self-reported past or current treatment for depression.  In the Health in Men Study, the OR of prevalent depression increased 4 % (OR, 1.04; 95 % CI: 1.02 to 1.05) with every unit increase of tHcy (micromoles per liter).  The tHcy was 0.19 mg/L higher among participants with the MTHFR C677T TT genotype compared with the CC genotype.  The meta-analysis showed that older adults with high tHcy had increased risk of depression (OR, 1.70; 95 % CI: 1.38 to 2.08) and TT carriers were 22 % more likely than CC carriers to have current depression or a history of depression (OR, 1.22; 95 % CI: 1.01 to 1.47).  The authors concluded that the triangular association between the MTHFR genotype, tHcy, and depression implies that higher concentrations of tHcy increase the risk of depression and that lowering tHcy by 0.19 mg/L could reduce the odds of depression by about 20 %.  They noted that confirmatory data from sufficiently powered randomized trials of Hcy-lowering therapy are now needed to test if the relationship between tHcy and depression is truly causal.

In a meta-analysis, McGimpsey and colleagues (2009) evaluated the role of plasma tHcy concentrations and homozygosity for the thermolabile variant of the MTHFR C677T gene as risk factors for retinal vein occlusion (RVO).  Data sources included MEDLINE, Web of Science, and PubMed searches and searching reference lists of relevant articles and reviews.  Reviewers searched the databases, selected the studies, and then extracted data.  Results were pooled quantitatively using meta-analytic methods.  Main outcome measures were tHcy concentrations and MTHFR genotype.  There were 25 case-control studies for tHcy (1,533 cases and 1,708 controls) and 18 case-control studies for MTHFR (1,082 cases and 4,706 controls).  The mean tHcy was on average 2.8 micromol/L (95 % CI: 1.8 to 3.7) greater in the RVO cases compared with controls, but there was evidence of between-study heterogeneity (p < 0.001, I(2) = 93 %).  There was funnel plot asymmetry suggesting publication bias.  There was no evidence of association between homozygosity for the MTHFR C677T genotype and RVO (odds ratio [OR] 1.20; 95 % CI: 0.84 to1.71), but again marked heterogeneity (p = 0.004, I(2) = 53 %) was observed.  The authors concluded that there was some evidence that elevated tHcy was associated with RVO, but not homozygosity for the MTHFR C677T genotype.  Both analyses should be interpreted cautiously because of marked heterogeneity between the study estimates and possible effect of publication bias on the tHcy findings.  They stated that because of the presence of heterogeneity and publication bias, no recommendation can be made with regard to routine investigation and treatment of elevated tHcy in the setting of RVO.

Oterino et al (2010) stated that it has been suggested that Hcy and the 5'-10'-MTHFR C677T variant are implicated in the pathogenesis of migraine.  Homocysteine has the potential to damage endothelium and accelerate atherosclerosis.  Genetic factors such as the MTHFR C677T polymorphism, and other polymorphisms in folate-related genes associated with high Hcy levels, may contribute to increasing this vascular risk.  These investigators recruited 427 migraine patients (199 without aura [MO]; 228 with aura [MA]), and 310 controls in a neurologic clinic.  Plasma Hcy levels and 6 polymorphisms corresponding to 6 folate-related genes, including the MTHFR C677T variant, were determined in all migraine subjects and in a subset of 155 controls.  These researchers found higher sex-adjusted Hcy levels in MA (mean of 11.02 microM) than MO patients (9.86 microM; p = 0.005 for the difference).  Plasma Hcy levels higher than 12.0 microM doubled the risk for MA (OR = 2.145; 95 % CI: 1.3 to 3.4; p = 0.001), and those higher than 15.0 microM incurred a 6-fold increase (OR = 5.95; 95 % CI: 2.1 to 20.0, p < 0.001).  The number of MTHFR 677T alleles was the best genetic predictor of Hcy levels (r(2) = 0.06; p = 6.2e-6; corrected for genetic variants analyzed) and this effect remained significant after correction for other confounding factors.  Using multi-dimensionality reduction approaches, these researchers observed significant epigenetic interaction among some of the folate-related genetic variants to predict higher Hcy levels, and also among higher Hcy levels and folate-related genetic variants to predict the end-diagnosis of MA only among migraineurs.  In controls, Hcy levels and the number of MTHFR 677T alleles were found to be intermediate between those observed in MA and MO patients.  The authors concluded that these findings suggest that MA patients have higher Hcy levels.  They also observed complex epigenetic interaction among folate-related enzymes, sex, and Hcy levels predicting MA phenotype.  Nevertheless, genetic factors explained only a minor proportion of the variance for both Hcy plasma levels and for predicting MA phenotype.  Determination of MTHFR C677T polymorphisms and Hcy levels may be useful to identify patients with a high risk of suffering from MA.

Spijkerman et al (2005) explored to what extent Hcy, S-adenosylmethionine (SAM), S-adenosylhomocysteine, total folate, 5-methyltetrahydrofolate (5-MTHF), vitamin B12, and vitamin B6 are associated with endothelium-dependent, flow-mediated vasodilation (FMD), and whether these associations are stronger in individuals with diabetes or other cardiovascular risk factors.  In this population-based study of 608 elderly people, FMD and endothelium-independent nitroglycerin-mediated dilation (NMD) were ultrasonically estimated from the brachial artery (absolute change in diameter [mum]).  High SAM and low 5-MTHF were significantly associated with high and low FMD, respectively (linear regression coefficient, 95 % CI: 48.57 microm (21.16 to 75.98) and -32.15 microm (-59.09 to -5.20), but high Hcy was not (-15.11 microm (-42.99 to 12.78).  High SAM and low 5-MTHF were also significantly associated with high and low NMD, respectively.  Nitroglycerin-mediated dilation explained the association of 5-MTHF with FMD but not of SAM.  No interactions were observed for diabetes or cardiovascular risk factors.  The authors concluded that in this elderly population, both SAM and 5-MTHF are associated with endothelial and smooth muscle cell function.  The effect of Hcy on endothelial function is relatively small compared with SAM and 5-MTHF.  The relative impact of SAM, 5-MTHF, and Hcy, and the mechanisms through which these moieties may affect endothelial and smooth muscle cell function need clarification.

In a double-blind, placebo-controlled, randomized clinical trial, Thompson et al (2009) examined if exogenous SAM (AdoMet) increases the level of plasma Hcy, a potential cardiovascular risk factor, in healthy human subjects.  A total of 52 healthy human volunteers were included in this study.  Subjects received placebo or AdoMet (800 mg per day) for 4 weeks; Hcy levels were measured before and after administration of AdoMet or placebo.  The primary outcome measure was change in Hcy level.  Secondary outcome measures included an interim Hcy determination (at 2 weeks) and changes in levels of high-sensitivity CRP (hsCRP), lipids, and alanine aminotransferase.  There was no statistically significant change in Hcy between groups.  Similarly, no statistically significant differences in change in Hcy or hsCRP levels were observed at 2 or 4 weeks.  There was a small but statistically significant increase (p < 0.04) in alanine aminotransferase at week 2 and a statistically significant decrease (p < 0.04) in total cholesterol in the AdoMet group compared with the placebo group.  The authors concluded that AdoMet at a daily dose of 800 mg for 4 weeks does not appear to significantly affect Hcy levels in the blood.

Hooshmand and colleagues (2010) examined the relation between serum levels of homocysteine (tHcy) and holo-transcobalamin (holoTC), the active fraction of vitamin B12, and risk of incident AD in a sample of Finnish community-dwelling elderly.  A dementia-free sample of 271 subjects aged 65 to 79 years derived from the Cardiovascular Risk Factors, Aging, and Dementia (CAIDE) study was followed up for 7 years to detect incident AD.  The association between serum tHcy and holoTC with AD was analyzed with multiple logistic regression after adjusting for several potential confounders, including common vascular risk factors.  The ORs (95 % CI) for AD were 1.16 (1.04 to 1.31) per increase of 1 μmol/L of tHcy at baseline and 0.980 (0.965 to 0.995) for each increase of 1 pmol/L baseline holoTC.  Adjustment for several potential confounders including age, sex, education, APOE-4 allele, body mass index, Mini-Mental State Examination, smoking, stroke, and blood pressure did not alter the associations: ORs (95 % CI) for AD became 1.19 (1.01 to 1.39) for tHcy and 0.977 (0.958 to 0.997) for holoTC.  Adjusting for holoTC attenuated the tHcy–AD link (OR changed from 1.16 to 1.10, 95 % CI: 0.96 to 1.25).  The holoTC–AD relationship was less influenced by controlling for tHcy (OR changed from 0.980 to 0.984, 95 % CI: 0.968 to 1.000).  Addition of folate did not change any of the results.  The authors concluded that these findings suggested that both tHcy and holoTC may be involved in the development of AD.  The tHcy–AD link may be partly explained by serum holoTC.  The role of holoTC in AD should be further investigated.  It is as yet unclear whether holoTC or Hcy is the key player in the association.  Careful examination of the evidence is needed to ascertain who is the perpetrator in the complex pathology of AD and other dementias.

In an editorial that accompanied the afore-mentioned study, Mangoni and Woodman (2011) stated that "[a]lthough informative, the data from Veeranna et al need to be interpreted with caution for a number of reasons ... Further studies are required to ascertain the role of Hcy concentration in risk reclassification in cerebrovascular disease, peripheral arterial disease, and heart failure .... If Hcy is to be used as a screening toll in primary prevention, it is imperative that further trials are conducted in low- and intermediate-risk patients without previous CVD.  Only then can the real value of measuring Hcy as a nontraditional risk factor or risk marker be quantified".

In a prospective controlled study, Gulhan and associates (2011) investigated serum Hcy and asymmetric dimethyl-arginine (ADMA) levels in patients with premature ovarian failure (POF).  A total of 69 women (32 with POF and 37 apparently healthy women) were included in the study.  Fasting blood samples were drawn to measure serum Hcy and ADMA levels using ELISA method.  The study and control group had a mean age of 37.3 +/- 2.6, 37.5 +/- 2.5 years; a mean Hcy level of 13.54 +/- 5.19, 12.71 +/- 3.99 mmol/L and a mean ADMA level of 1.32 +/- 0.27, 1.26 +/- 0.36 mmol/L, respectively.  There were no statistically significant differences between the 2 groups in terms of Hcy and ADMA levels (p values of 0.465 and 0.423, respectively).  A negative significant correlation was found between estradiol and ADMA (p < 0.05).  The authors concluded that Hcy and ADMA levels did not change in comparison with the control group, which suggested that estrogen deficiency in patients with POF does not have any effect on Hcy and ADMA levels.

Pinna et al (2012) determined the plasma levels of the sulfur-containing amino acids Hcy, cysteine, cysteinylglycine, glutamylcysteine, glutathione and taurine in patients with BRVO and in healthy subjects and examined if there are statistically significant differences between patients and controls.  Homocysteine, cysteine, cysteinylglycine, glutamylcysteine, glutathione and taurine plasma levels were measured in 40 patients with BRVO and 80 age- and gender-matched control subjects by using laser-induced fluorescence capillary electrophoresis methods.  Wilcoxon's or Student's t test was used, when appropriate, to determine differences between the groups.  Conditional logistic regression analysis was performed to determine the risk factors for BRVO.  Branch retinal vein occlusion patients showed significantly lower plasma concentrations of cysteinylglycine (p = 0.02) and taurine (p < 0.0001) than controls.  Conversely, there were no significant differences in plasma Hcy, cysteine, glutamylcysteine and glutathione between patients with BRVO and controls.  Conditional logistic regression analysis revealed an odds ratio of 0.95 (95 % CI: 0.92 to 0.98, p = 0.001) for taurine and 0.86 (95 % CI: 0.78 to 0.96, p = 0.006) for cysteinylglycine.  The authors concluded that the findings of this study failed to demonstrate an association between BRVO and the plasma levels of Hcy, cysteine, glutamylcysteine and glutathione.  Cysteinylglycine and taurine were significantly lower in BRVO patients, thus suggesting that reduced plasma levels of these sulfur-containing amino acids may contribute to the pathogenesis of BRVO.

Furthermore, there is insufficient evidence on the role of Hcy in any of the following conditions (not an all-inclusive list):

• Acquired thrombophilia
• Autism
• Down's syndrome
• Gaucher's disease
• HELLP syndrome
• Meniere's disease
• Methotrexate therapy
• Monitoring response to vitamin B-12 therapy
• Movement disorders (e.g., Huntington's disease, Parkinson's disease, and primary dystonia)
• Primary carnitine deficiency
• Pulmonary hypertension.

Role of Hcy in the Management of Celiac Disease/Inflammatory Bowel Disease

Ruisi and colleagues (2015) noted that Crohn's disease and ulcerative colitis are both systemic chronic diseases that alter bowel physiology. The central process in inflammatory bowel disease (IBD) and the associated manifestations are the result of B-cell production of IgG autoantibodies directed against self-antigens in various organ systems including coronary endothelium.  Previous studies have demonstrated significant micro-vascular endothelial dysfunction in patients with IBD compared to patients not affected by the disease.  Thee investigators analyzed the relation, if any, between IBD and the development of premature coronary artery disease (CAD).  They queried their hospital database to find IBD patients admitted to the hospital from January 1, 2007 to December 31, 2008.  Patients with traditional cardio-vascular (CV) disease risk factors including hypertension, congestive heart failure (CHF), diabetes, aged greater than or equal to 65 years, hyperlipidemia, family history, end-stage renal disease (ESRD), and greater than 5 pack-year smoking history were excluded from the study cohort.  The charts of the remaining 300 patients with diagnosed IBD were then analyzed for the incidence of CV disease events including acute myocardial infarction (MI), unstable angina, positive stress testing, and any cardiac intervention including coronary angioplasty and/or intra-coronary stent implantation.  Of the 300 patients included, only 1 patient had a CV disease event.  This patient had a positive exercise stress thallium test.  Otherwise, the remaining 299 patients (99.7 %) did not have any reported CV disease events over the 2-year follow-up period.  The authors concluded that most of the clinical sequelae of CV disease events are the result of inflammatory changes at the vascular level.  While IBD is associated with a chronic inflammatory state as reflected by high sedimentation rates, CRP, Hcy levels, etc., the data appeared to indicate that chronic inflammation in the absence of traditional risk factors is not associated with an increased risk of premature CV disease events.  They stated that more wide-scale prospective studies should be performed to elucidate the relationship, if any, between chronic inflammation and CV disease risk.

Peterson and Grossman (2016) stated that although many people have symptoms of celiac disease (CD), it can take a while to diagnose. Villous atrophy may be present long before any gastro-intestinal (GI) symptoms.  An important point to acknowledge is that CD could be identified earlier in some women with a positive family history; CD also could be the cause of some women's reproductive problems.  Primary care providers, using comprehensive history taking, are in the unique position to identify individuals who may have CD, assist women in gaining knowledge about a gluten-free diet, order diagnostic testing, and refer to a gastroenterologist.  The positive change in fertility with a simultaneous improvement of nutrient deficiencies shortly after adopting a gluten-free diet indicates a possible link between such nutrients and sex hormone function.  High levels of Hcy, which can negatively impact fertility, have also been linked to individuals with problems, such as CD, that decrease vitamin B12 absorption.  

Furthermore, UpToDate reviews on “Management of celiac disease in adults” (Ciclitira, 2016), “Overview of the medical management of mild to moderate Crohn disease in adults” (Farrell and Peppercorn, 2016 ), and “Management of mild to moderate ulcerative colitis in adults” (MacDermott, 2016) do not mention homocysteine testing as a management tool.

Role of Hcy in Age-Related Macular Degeneration

Pinna and colleagues (2018) noted that there is still no agreement on tHcy role in age-related macular degeneration (ARMD).  These researchers performed a systematic review and meta-analysis of the published data on the correlation between tHcy and ARMD.  Medline/PubMed and ISI Web of Sciences searches were performed according to MOOSE guidelines.  Case-control studies were eligible for inclusion.  Participants and controls were ARMD patients and subjects without ARMD.  The main outcome measure was wet ARMD; Hcy level was the main exposure variable.  Data were pooled using a random-effects model.  A total of 12 case-control studies were identified: 10 assessed wet ARMD, 4 dry ARMD, 1 early ARMD, 1 late ARMD, and 1 any ARMD.  As for wet AMD, there was a total of 453 cases and 514 controls.  Mean tHcy was on average 1.1 μmol/L (95 % CI: 0.96 to 1.25) greater in wet ARMD cases, but there was evidence of extreme between-study heterogeneity (p < 0.001, I2  = 91.8 %).  In a model homogenous for age, including 6 wet ARMD studies (214 cases, 274 controls), mean tHcy was on average 0.58 μmol/L (95 % CI: 0.35 to 0.73) greater in the case group, a not statistically significant result (p = 0.144) associated with moderate heterogeneity (I2  = 39.2 %).  The authors concluded that the findings of this meta-analysis indicated that there was some weak evidence that increased tHcy might be associated with wet ARMD; however, this result should be interpreted cautiously, because of a marked between-study heterogeneity and the possible effect of publication bias.  They stated that future studies, preferably of cohort design, are needed before any firm conclusions on the putative role of increased tHcy on ARMD can be drawn.

Role of Hcy in Alzheimer's Disease

Janel and colleagues (2017) stated that early identification of Alzheimer's disease (AD) risk factors would aid development of interventions to delay the onset of dementia, but current biomarkers are invasive and/or costly to assess.  Validated plasma biomarkers would circumvent these challenges.  These researchers previously identified the kinase DYRK1A in plasma.  To validate DYRK1A as a biomarker for AD diagnosis, these investigators measured the levels of DYRK1A and the related markers brain-derived neurotrophic factor (BDNF) and Hcy in 2 unrelated AD patient cohorts with age-matched controls.  Receiver-operating characteristic curves and logistic regression analyses showed that combined assessment of DYRK1A, BDNF and Hcy had a sensitivity of 0.952, a specificity of 0.889 and an accuracy of 0.933 in testing for AD.  The blood levels of these markers provided a diagnosis assessment profile.  Combined assessment of these 3 markers out-performed most of the previous markers and could become a useful substitute to the current panel of AD biomarkers.  The authors concluded that these results associated a decreased level of DYRK1A with AD and challenged the use of DYRK1A inhibitors in peripheral tissues as treatment.  The role of Hcy as a diagnostic marker for AD needs to be further investigated.

Role of Hcy for Assessment of Lung Transplant Candidates

UpToDate reviews on “Lung transplantation: An overview” (Hachem, 2018a) and “Lung transplantation: General guidelines for recipient selection” (Hachem, 2018b) do not mention measurement of homocysteine as an approach for management of lung transplant candidates.

Role of Hcy as a Biomarker for the Development and/or Progression of Erectile Dysfunction

Sansone and colleagues (2018) noted that elevated levels of serum homocysteine (Hcy) have been associated with cardiovascular diseases and endothelial dysfunction, conditions closely associated with ED.  In a meta-analysis, these investigators examined serum Hcy levels in subjects with ED compared to controls in order to clarify the role of Hcy in the pathogenesis of ED.  Medline, Embase, and the Cochrane Library were searched for publications investigating the possible association between ED and Hcy.  Results were restricted by language, but no time restriction was applied.  Standardized mean difference (SMD) was obtained by random effect models.  A total of 9 studies were included in the analysis with a total of 1,320 subjects (489 subjects with ED; 831 subjects without ED).  Pooled estimate was in favor of increased Hcy in subjects with ED with a SMD of 1.00, 95 % CI: 0.65 to 1.35, p < 0.0001.  Subgroup analysis based on prevalence of diabetes showed significantly higher SMD in subjects without diabetes (1.34 (95 % CI: 1.08 to 1.60)) compared to subjects with diabetes (0.68 (95 % CI: 0.39 to 0.97), p < 0.0025 versus subgroup without diabetes).  The authors concluded that findings from this meta-analysis suggested that increased levels of serum Hcy were more often observed in subjects with ED.  They stated that based on existing literature on this topic, a causative role for hyperhomocysteinemia as an independent risk factor for ED could be postulated, although confirmation would require interventional studies aimed to decrease serum Hcy levels considering erectile function as primary outcome.  These researchers stated that actually, only in rat model of hyperhomocysteinemia has been observed an improvement in erectile function after being treated with a demethylation agent.  These investigators also reported significantly higher levels of Hcy in subjects without diabetes, compared to diabetic men.  They noted that while one could assume that this is further proof of a multi-factorial pathogenesis for ED, it is also a clear indication that future research in this field should examine the possible association with other known risk factors such as smoking habit and obesity in order to adequately address the possible effects of different variates.

The authors stated that this study has several drawbacks, most notably the small number of studies (n = 9) involved and the lack of a clear definition of ED.  A single study assessed presence of ED by means of a single question (“How would you describe your ability to get and keep an erection that is adequate for satisfactory intercourse?”).  The remaining studies used validated questionnaires: in detail, 4studies used the IIEF and 4 studies used the IIEF-5.  However, most studies did not report separate measurements of serum Hcy based on the degree of severity of ED.

Role of Hcy for Assessment of Central Retinal Vein Occlusion

Backhouse et al (2000) carried out a pilot study on the prevalence of familial thrombophilia in all cases of retinal vein occlusion with no known risk factors.  Over the 1-year study period, a total of 71 patients presented with a new diagnosis of retinal vein occlusion (age of 28 to 90 years); 55 (77 %) were excluded because of local predisposing factors.  The remaining 16 (23 %) had a full risk factor history taken and blood investigations of rheological factors and thrombophilia including tests for the factor V Leiden mutation, prothrombin G20210A allele and hyperhomocystinemia.  Of those with no local predisposing factors, 3 patients had anti-phospholipid antibodies, 3 had raised fibrinogen levels, 4 had hyperhomocystinemia and 1 was heterozygous for the Leiden mutation.  Other lifestyle risk factors such as obesity, smoking and a positive family history of venous thrombosis were not uncommon.  No patient had the prothrombin G20210A variant.  The authors concluded that it appeared likely that several risk factors, both genetic and acquired, need to be present for thrombosis to occur.  In examining a new patient with a retinal vein occlusion, one should test for hypertension, glaucoma and diabetes mellitus.  Estimation of plasma viscosity and a full blood count are cheap examinations that may reveal neoplasia or vasculitis, and lipid levels should be estimated.  In a young patient or one with an unexpected vein occlusion and a personal or family history of thrombosis, a hypercoagulable state may rarely be identified.  This additional testing should include testing for anti-phospholipid antibodies and a full thrombophilia screen including the factor V Leiden mutation, homocysteine and the prothrombin variant as part of a clinical trial.  Until the role of these markers in thrombosis is better defined in relation to causation of retinal vein occlusion and treatment has been shown to improve outcome, these investigators could not recommend them for routine testing.  If a hereditary defect was found, referral should be made to a hematologist and consideration given to anti-coagulation and screening of family members to prevent further thrombotic episodes.  Retinal vein occlusions are multi-factorial in origin except in rare cases.

In a prospective, observational, case-series study with retrospective comparative controls, Lahey et al (2002) examined if hypercoagulability plays a role in thrombus formation in patients with central retinal vein occlusion (CRVO) who are less than 56 years of age.  Subjects included 55 consecutive patients with CRVO less than 56 years of age.  The laboratory's age-matched control groups were used to compare results for the same tests.  A total of 55 patients with CRVO less than 56 years old (mean age of 44 years) underwent laboratory evaluation for homocysteine, activated protein C resistance, protein C activity, protein S activity, antithrombin III activity, anti-phospholipid antibodies, and anticardiolipin antibodies.  The results were compared with previously drawn age-matched control groups obtained by the same laboratory for statistical significance.  Patients were considered to have a positive test if their results were outside the laboratory's established range.  A total of 15 of 55 patients (27 %) had 1 positive test result suggesting hypercoagulability.  Compared with the control groups, these patients of less than 56 years old with CRVO had a higher incidence of coagulation abnormalities by laboratory testing.  Among the parameters tested, hyperhomocysteinemia and circulating anti-phospholipid antibodies were significantly more common in the CRVO patients (p < 0.05) compared with age-matched controls.  The authors concluded that hypercoagulability may play a role in the pathogenesis of CRVO in patients less than 56 years old.  The cause of CRVO remains multi-factorial, and laboratory tests suggesting hypercoagulable states alone could not account for the cause in most of these patients less than 56 years of age.  The authors recommended examining blood pressure (BP), intra-ocular pressure (IOP), complete blood count (CBC), glucose levels, and a lipid panel on all patients with CRVO.  When tests for these common risk factors for CRVO are negative, consider ordering selected tests in young patients with CRVO to rule out thrombophilia, especially in patients with bilateral CRVO, a history of previous thrombosis, or family history of thrombosis.

Brown et al (2002) noted that recent studies have shown a relationship between elevated levels of homocysteine and vascular disease including cerebrovascular accidents (CVAs) and myocardial infarctions (MIs).  In an age- and gender-matched case-controlled study, these researchers examined patients with a recent retinal vein occlusion to determine if there was an associated elevation of homocysteine.  A total of 20 patients with retinal venous occlusive disease within the previous 6-month period and 40 control patients without retinal venous occlusive disease were enrolled in the study.  Subjects underwent laboratory testing to determine serum homocysteine levels.  Furthermore, a health survey was completed and several laboratory tests relating to vascular disease including cholesterol were checked.  Two control groups were selected -- the 1st group included patients with diabetes and no history of retinal vascular occlusive disease; while the 2nd control group included non-diabetic patients who also had no prior history of retinal vascular occlusive disease.  The main parameter measured in this study was fasting homocysteine.  A total of 15 of 20 patients (75 %) with retinal vein occlusion disease had an elevated fasting serum homocysteine level, whereas only 5 of 40 control patients (13 %) had an abnormal homocysteine level (p < 0.0005).  The authors concluded that patients with retinal venous occlusive disease had higher levels of homocysteine, which may serve as a modifiable risk factor.

Agarwal and Biswas (2009) reported a case of a 20-year old healthy patient with CRVO in his right eye attributable to primary anti-phospholipid antibody syndrome.  The subject had poor vision in the right eye and diffuse retinal hemorrhages was examined for infections, autoimmune disease, diabetes, and hypertension.  Testing for homocysteine, anticardiolipin antibodies, lupus anticoagulant, and functional assays for protein S and protein C was carried out to detect a hypercoagulable state.  Laboratory investigations revealed elevated levels of IgG and M anticardiolipin antibodies.  Long-term oral anti-coagulants were given to reduce the risk of future thromboses.  The authors concluded that anti-phospholipid antibodies play an important role in occlusive retinal vascular disorders, especially in young patients without any conventional risk factors.

Furthermore, an UpToDate review on “Retinal vein occlusion: Epidemiology, clinical manifestations, and diagnosis” (Han and Ahmad, 2021) states that “We screen patients for cardiovascular risk factors by obtaining a fasting glucose or hemoglobin A1C and a fasting lipid panel.  In addition, we perform a hypercoagulable workup including testing for factor V Leiden mutation, homocysteine, and anticardiolipin antibody in patients with a personal or family history suggesting a hypercoagulable state, and in patients under age 50 who do not have strong evidence of arteriosclerotic risk factors”.

Role of Hcy for Pregnant Member with Mildly Elevated Anticardiolipin IgM

Spegiorin et al (2010) examined the prevalence of high anticardiolipin antibody and lupus anticoagulant levels in women with histories of at least 2 miscarriages.  These researchers evaluated the presence of high anticardiolipin antibody and lupus anticoagulant levels in 52 patients aged between 17 and 41 years (mean of 26.7) who had histories of repeated miscarriages.  High anticardiolipin antibody levels were identified in 55.77 % and lupus anticoagulant levels in 2 % of the individuals.  The authors concluded that women who suffered from repeated miscarriages presented with a high prevalence of anticardiolipin antibodies.  Moreover, these researchers stated that further studies are needed as these antibodies are associated with hypercoagulability; thus, increasing the risk of thromboembolic events.

GeneReviews’ webpage on “Prothrombin Thrombophilia” (Kujovich, 2021) stated that no consensus exists on the optimal management of prothrombin thrombophilia during pregnancy; guidelines are derived from studies in non-pregnant individuals.  The author also stated that measurement of plasma concentration of homocysteine is not recommended since no data support a change in duration of anti-coagulation or the use of vitamin supplementation in individuals with hyperhomocysteinemia and a history of VTE.

Furthermore, an UpToDate review on “Inherited thrombophilias in pregnancy” (Lockwood and Bauer, 2022) states that “Tests to avoid -- We recommend not performing the following tests as part of thrombophilia screening: homocysteine level, methylenetetrahydrofolate reductase (MTHFR) polymorphism (C677T, A1298C) testing, plasminogen activator inhibitor-1 (PAI-1) polymorphism testing, or factor VIII level … Pregnant patients are routinely prescribed multivitamins containing folic acid to reduce the risk of neural tube defects.  High homocysteine levels are a weaker risk factor for VTE or arterial thrombosis than reported in early studies (odds ratio 1.2-1.6 versus 2.0) and may not be an independent risk factor at all.  Furthermore, intervention studies with B vitamin supplementation, albeit in nonpregnant patients, do not show a reduction in arterial or venous thrombotic events”.

As a Biomarker in Hematological Complications of COVID-19 Infection

Ahmed and Noori (2023) examined the significance of Hcy as a biomarker for COVID-19 infection, and the relation of Hcy with COVID-19 severity in obese people and diabetic patients.  The study groups were COVID-19 patients + Diabetic + Obese (CDO); COVID-19 patients + Diabetic (CD), COVID-19 patients + Obese (CO), and Healthy Group (HG).  Serum levels of homocysteine, IL-6, D-dimer, vitamin B12, and folate were measured by a fully automated biochemistry device Cobas 6000 analyzer series.  The mean serum concentration of Hcy in the COD, CD, CO and H groups were 32.0114, 23.604, 19.4154, and 9.3206 uM/L, respectively.  The mean concentration of Hcy levels between every 2 groups was statistically significant differences (p < 0.05) except for the CD and the CO group (p = 0.957).  In the CDO group, the males have higher mean concentrations than females (p < 0.05).  The means of Hcy concentrations in the CDO group among different age groups were different (p < 0.001).  The serum Hcy level in the CDO group has a strong positive correlation (r = 0.748) with D-dimer and a strong negative correlation (r = - 0.788) with serum folate, while its correlation with serum vitamin B12 is moderately negative (r= -0.499) and its correlation with serum IL-6 was weakly positive (r = 0.376).  The AUC value for Hcy in predicting COVID-19 in the CDO group was 0.843, while 0.714 for the CD group, and 0.728 for the CO group.  The serum Hcy concentration test for all study groups was compared to the serum IL-6 test and the sensitivity was equal to 95 % and its specificity was 67.5 %.  The authors concluded that serum Hcy has potential predictive power in COVID-19 patients, and the severity of COVID-19 infection and the type of co-morbidity is associated with higher sensitivity and specificity of Hcy serological tests.

As a Biomarker in Carotid Artery Stenosis with Coronary Artery Disease

Li et al (2023b) stated that patients with left carotid artery atherosclerotic stenosis have an increased ischemic stroke risk.  Left carotid stenosis, the most common cause of the transient ischemic attack (TIA), is related to a higher risk of acute stroke.  Left carotid artery stenosis is also associated with cerebral artery infarction.  The significant coronary stenosis promotes ST-segment elevation MIs.  The severe coronary stenosis plays an important role in development and progression of MI.  However, the dynamic changes of circulating oxidative stress and inflammatory markers in the carotid stenosis combined with coronary artery stenosis are unclear, and it also remains unknown whether mark of oxidative stress and inflammation are potential therapeutic targets for carotid stenosis combined with coronary artery stenosis.  These researchers examined the effects of oxidative stress combined with an inflammatory response on left carotid artery stenosis in CAD patients.  They tested the hypothesis that levels of markers of oxidative stress and inflammation are associated with co-existent severe carotid and coronary artery stenosis in patients.  These investigators measured the circulating levels of malondialdehyde (MDA), oxidized low-density lipoprotein (OX-LDL), Hcy, F2- isoprostanes (F2-IsoPs), TNFr-alpha (TNF-α), high-sensitivity CRP (hs-CRP), prostaglandin E2 (PG-E2) and interferon-gamma (IFN-γ) in patients with combined carotid and coronary artery severe stenosis.  They also examined the relationships among oxidative stress, inflammation, and severe stenosis of the carotid with a coronary artery in patients.  Levels of MDA, OX-LDL, Hcy, F2-IsoPs, TNF-α, hs-CRP, PG-E2, and IFN-γ were remarkably increased (p < 0.001) in patients with combined carotid and coronary artery severe stenosis.  High levels of oxidative stress and inflammation may be related to severe stenosis of the carotid with coronary arteries in patients.  The authors concluded that these findings indicated that measurements of oxidative stress and inflammatory markers may be valuable for the assessment of the degree of carotid with coronary artery stenosis.  The biomarkers of oxidative stress and inflammatory response may become therapeutic targets for carotid artery stenosis with coronary artery stenosis in patients.

The authors stated that this study had several drawbacks.  First, this was a small, single-center study.  There was no more clinical information available regarding the effects of co-morbidities (e.g., primary headache disorders, hypertension, type II diabetes, amaurosis fugax, dizziness, and TIA) on the inflammatory response and oxidative stress in patients with severe stenosis of the carotid and coronary arteries.  Second, this study was also limited in lacking evaluation of the effects of alcohol drinking on biomarkers of inflammation and oxidative stress in patients with severe stenosis in the carotid and coronary arteries.  Third, the lack of data analysis regarding the effects of cigarette smoking on the levels of inflammatory mediators and oxidative stress biomarkers in patients with carotid and coronary arteries severe stenosis.  It could show more clinical information if these researchers would examine the characterization of atheromatous plaques in carotid and coronary artery lesions.

As a Biomarker in Diabetic Nephropathy and Diabetic Retinopathy

In a cross-sectional study, Li et al (2023a) examined the association of Hcy with diabetic nephropathy (DN) and diabetic retinopathy (DR) in a representative U.S. population.  This trial employed data from participants in the National Health and Nutrition Examination Survey 2005 to 2006.  Metrics including Hcy level, urinary albumin to creatinine ratio, estimated glomerular filtration rate (eGFR), and retinopathy grading were collected.  Multiple logistic regression models were used to examine the association of Hcy with DN and DR.  A total of 630 participants were included in this study.  The Hcy level was significantly higher in those with DN and DR than those without DN and DR.  Hcy was associated with an increased risk of DN (OR = 1.31, 95 % CI: 1.18 to 1.46; p < 0.001).  In the fully adjusted model of DN (model II), compared to participants in quartiles 1 of Hcy, the adjusted ORs for participants in quartiles 2 to 4 were 1.49 (95 % CI: 0.52 to 4.26; p = 0.426), 3.81 (95 % CI: 1.35 to 10.73; p = 0.015), and 14.08 (95 % CI: 3.84 to 51.66; p = 0.001), respectively.  Hcy was also associated with an increased risk of DR (OR = 2.260, 95 % CI: 1.212 to 4.216; p = 0.014); however, this association was non-significant in the fully adjusted model of DR (model II).  The authors concluded that in diabetic patients, Hcy was associated with increased risk of DN in a non-linear manner.  Furthermore, Hcy was associated with the risk of DR; however, the association was attenuated after adjusting for confounders.  Moreover, these researchers stated that in the future, Hcy can potentially be used as an early screening indicator for diabetic microvascular complications.

As a Biomarker in for Risk of Pre-Eclampsia

In a meta-analysis, Zhang et al (2022) examined the correlation between blood Hcy levels and pre-eclampsia (PE) risk in pregnant women.  Related articles were searched using PubMed, Embase, and Web of Science databases.  Methodological quality of included studies was examined using the Newcastle-Ottawa Quality Assessment Scale (NOS).  Cochran's Q and I2 tests were used to evaluate heterogeneity; and Egger's test was used to evaluate publication bias.  A sensitivity analysis was carried out to test stability of the results using a 1-by-1 elimination method.  Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) was used to examine certainty of evidence.  A total of 9 studies (4,384 PE and 26,021 non-PE patients) were included in the meta-analysis.  The methodology of them was of good quality, with NOS scores of 5 to 8; however, there was a significant heterogeneity among included studies.  Thus, the random effect model was generated; and combined results suggested a significant association between increased level of Hcy in pregnant women and PE risk.  Although a significant publication bias was found in the current study with a p value of 0.006 in the Egger test, sensitivity analysis showed that the combined results were stable and did not vary significantly from any single study.  However, the GRADE evidence quality was very low, which may lower the recommendation of pooled results.  The authors concluded that increased levels of Hcy in maternal blood were significantly associated with the risk of PE; however, low certainty of evidence need to be improved by more high-quality studies.

Identification of Individuals with High Arterial Thrombosis Risk

Eldibany and Caprini (2007) stated that Hcy, a sulfur-containing amino acid, absent in natural diets, is a metabolic intermediary in trans-methylation and trans-sulfuration reactions.  Such reactions are essential to normal cellular growth, differentiation, and function.  Excess Hcy is associated with vascular disease and related disorders.  These investigators reviewed Hcy metabolism, the pathogenesis and classification of hyperhomocysteinemia, and the published literature examining the association of Hcy and MTHFR defects with arterial and venous thromboembolism and related disorders.  In addition, they discussed the role of vitamin supplementation in patients with hyperhomocysteinemia.  Articles addressing the objectives were selected and reviewed; pertinent studies and conclusions were summarized, grouped, and contrasted.  The authors concluded that the association of hyperhomocysteinemia and arterial and venous thrombosis is controversial.  Severe hyperhomocysteinemia is associated with atherosclerosis.  The effect of mild hyperhomocysteinemia is less certain.  Co-inheritance of MTHFR defects and factor V Leiden is likely to increase the risk of venous thromboembolism.  The association of MTHFR defects combined with no additional thrombophilic risk factors with venous thrombosis is less clear.  High doses of folic acid to lower Hcy levels might not be necessary.

Di Minno et al (2010) noted that the correlation between Hcy and vascular disease has been examined in several clinical studies that showed that elevation of plasma tHcy was an independent risk factor for atherosclerotic disease.  Major advances of Hcy metabolism disorders have been made during the last few years, entailing the rare homozygous enzyme deficiencies, as well as more common milder abnormalities.  In experimental and clinical studies, a Hcy-mediated oxidant stress has been reported to trigger platelet activation, which in turn would result in a tendency to thrombosis, in patients with severe hyperhomocysteinemia.  Similarly, the hypo-methylation hypothesis on acquired hyperhomocysteinemia (chronic renal disease) and the inter-relationship between hyperhomocysteinemia and impaired fibrinolysis, have added further biological plausibility to the role for hyperhomocysteinemia in vascular medicine.  However, whether hyperhomocysteinemia is causal or a marker of vascular disease, and whether plasma tHcy is only an indicator of the metabolic status remains to be clarified.  The role of the intake of some vitamins (e.g., folic acid, vitamin B6, and vitamin B12) on cardiovascular disease (CVD) is poorly understood -- in spite of the lowering of Hcy levels, vitamin supplementation failed to exert significant effects on cardiovascular risk.  On the other hand, although some lipid-modifying treatments increased Hcy levels in diabetics, there is no evidence that this attenuated the beneficial effects of such treatments on the cardiovascular risk.  The authors concluded that because of these uncertainties in this field, the available evidence did not provide support for routine screening and treatment for elevated Hcy to prevent CVD.

May and Moll (2021) stated that arterial thrombotic events in younger patients without a readily apparent etiology present significant diagnostic and management challenges.  These investigators presented a structured approach to diagnosis with consideration of common causes, including atherosclerosis and embolism, as well as uncommon causes, including medications and substances, vascular and anatomic abnormalities, systemic disorders, and thrombophilia.  They highlighted areas of management that have evolved within the past 5 years, including the use of dual-pathway inhibition in atherosclerotic disease, anti-thrombotic therapy selection in embolic stroke of undetermined source and left ventricular thrombus, the role of closure of patent foramen ovale for secondary stroke prevention, and the thrombotic potential of coronavirus disease 2019 (COVID 19) infection and vaccination.  The authors concluded with a representative case to illustrate the use of the diagnostic framework and discussed the importance of consideration of bleeding risk and patient preference in determining the appropriate management plan.  Moreover, these investigators did not mention Hcy testing as a management option.

Furthermore, an UpToDate review on “Overview of homocysteine” (Rosenson et al, 2024) states that “Elevated homocysteine levels have been associated with an increased risk of cardiovascular and cerebrovascular disease.  However, unlike modifiable risk factors such as hypertension, hypercholesterolemia, smoking, and diabetes, it has not been shown that lowering homocysteine prevents cardiovascular events … In general, we only measure serum homocysteine in patients suspected of having homocystinuria but not in other clinical settings”.

Prediction of Suicidal Behaviors in Patients with Depression

Kim et al (2023) noted that predictive values of multiple serum biomarkers for suicidal behaviors (SBs) have rarely been tested.  These researchers developed a panel of multiple serum biomarkers for predicting SBs in outpatients receiving a 12-month pharmacotherapy program for depressive disorders.  At baseline, 14 serum biomarkers and socio-demographic/clinical characteristics including previous suicidal attempt and present suicidal severity were evaluated in 1,094 patients with depressive disorders without a bipolar diagnosis.  Of these, 884 were followed for increased suicidal severity and fatal/non-fatal suicide attempt outcomes over a 12-month treatment period.  Individual and combined effects of serum biomarkers on these 2 prospective SBs were estimated using logistic regression analysis after adjustment for relevant co-variates.  Increased suicidal severity and fatal/non-fatal suicide attempt during the 12-month pharmacotherapy were present in 155 (17.5 %) and 38 (4.3 %) participants, respectively.  Combined cortisol, total cholesterol, and folate serum biomarkers predicted fatal/non-fatal suicide attempt, and these with interleukin-1 beta (IL-1β) and Hcy additionally predicted increased suicidal severity, with clear gradients robust to adjustment (p < 0.001).  The authors concluded that the use of multiple serum biomarkers could improve the predictability of SBs during the outpatient treatment of depressive disorders, potentially highlighting the need for more frequent monitoring and risk appraisal.  Moreover, these investigators stated that Hcy, which is involved in methylation reactions as with folate, has been associated with depressive disorders; however, it has not been examined as a biomarker for suicidality.  The significant findings on increased suicidal severity with Hcy should be considered as empirical, since the statistical significance was found in just 1 of 4 SBs and it was novel.

Prediction/Diagnosis of Post-Stroke Depression

In a meta-analysis, Tao et al (2023) examined the correlation between circulating Hcy and lipoprotein-associated phospholipase A2 (Lp-PLA2) levels and post-stroke depression (PSD).  Chinese (Chinese National Knowledge Infrastructure, Wanfang, and VIP) and English (PubMed, Embase, Medline, and Cochrane Library) databases on the correlation between circulating Hcy and Lp-PLA2 and PSD were collected.  Meta-analysis was carried out to compare the distinctions in circulating Hcy and Lp-PLA2 levels between PSD and non-PSD groups.  Meta-analysis was performed by using STATA 15.0 software.  A total of 20 studies were included in this meta-analysis.  The level of circulating Lp-PLA2 in the PSD group was higher than that in the non-PSD group (weighted mean differences [WMD]: 2.75, 95 % CI: 0.10 to 5.39, p = 0.002), which was an independent predictor of PSD (effect size = 0.05, 95 % CI: 0.03 to 0.07, p < 0.001).  The level of circulating Hcy in the PSD group was higher than that in the non-PSD group (WMD = 1.41, 95 % CI: 1.01 to 1.81, p < 0.001), which was an independent influencing factor for the occurrence of PSD (effect size = 0.07, 95 % CI: 0.04 to 0.09, p = 0.011).  The authors concluded that circulating Hcy and Lp-PLA2 levels were linked to the development of PSD, and could be applied as predictive or diagnostic indicators.

These researchers stated that this meta-analysis had several drawbacks.  First, the literature linked to Lp-PLA2 was insufficient, with only 3 studies, and more studies are needed to provide evidence to support it.  Second, most of the studies were carried out in mainland China; therefore , the available evidence is not representative enough.  Third, the studies included in the correlation analysis of Hcy and PSD exhibited publication bias.

References