Add-On Homocysteine
The Add-On Homocysteine test measures total plasma homocysteine, a sulphur amino acid intermediate in methionine metabolism influenced by folate, vitamin B12, vitamin B6, renal function, and MTHFR polymorphisms. It is ordered to investigate unexplained thrombosis, assess B-vitamin sufficiency, and screen for homocystinuria, though B-vitamin trials have not confirmed cardiovascular benefit from…
1
Biomarkers
Blood (serum)
Sample
5–7 business days
Turnaround
Yes
Fasting
What is the Homocysteine test?
The Add-On Homocysteine test measures total plasma homocysteine, an intermediate in one-carbon and sulphur metabolism whose concentration is governed by folate, vitamin B12, vitamin B6, renal clearance, and genetic variants. Although randomised trials failed to show that lowering homocysteine with B-vitamins reduces cardiovascular events, practitioners order the test to screen for nutritional deficiencies, to investigate unexplained venous or arterial thrombosis, and to identify severe elevations that signal homocystinuria or other inborn errors of metabolism. In cardiovascular risk assessment homocysteine occupies a contested space: observational associations are robust, but intervention data are negative. The test is therefore positioned as an adjunctive investigation in individuals with premature vascular disease or a strong family history where traditional risk factors do not fully explain the clinical picture, and as a functional marker of one-carbon pathway sufficiency when B-vitamin status is in question.
What does the Homocysteine test measure?
Homocysteine is a sulphur-containing amino acid formed during methionine metabolism. It sits at a metabolic crossroads: remethylation to methionine requires folate and vitamin B12, while transsulphuration to cysteine depends on vitamin B6. Elevated plasma homocysteine (hyperhomocysteinaemia) reflects impaired flux through one or both pathways and is influenced by nutritional status, genetic polymorphisms — notably MTHFR C677T — renal function, and certain medications.
Homocysteine gained attention as a putative independent cardiovascular risk factor, with observational data linking elevated concentrations to atherosclerosis, venous thromboembolism, and stroke. Though B-vitamin supplementation trials did not reduce cardiovascular events, homocysteine remains a marker of metabolic sufficiency in folate, B12, and B6 pathways and is used in select clinical scenarios: unexplained thrombosis, suspected inherited metabolic disorders, and assessment of nutritional repletion in malabsorption or dialysis. Severe hyperhomocysteinaemia flags homocystinuria.
Why is the Homocysteine test ordered?
- Evaluation of unexplained venous thromboembolism, arterial thrombosis, or recurrent pregnancy loss, particularly in the absence of conventional prothrombotic risk factors
- Investigation of premature atherosclerotic cardiovascular disease (onset before age 55 in men, 65 in women) or a strong family history, when routine lipid and glycaemic profiles are unrevealing
- Assessment of folate, vitamin B12, and vitamin B6 nutritional sufficiency in patients with malabsorption, chronic kidney disease, strict vegan diets, or following bariatric surgery
- Screening for homocystinuria in children or young adults presenting with lens dislocation, skeletal abnormalities, developmental delay, or unexplained thrombosis
- Monitoring patients on methotrexate, certain antiepileptic drugs, or other medications known to interfere with folate metabolism
- Functional-medicine evaluation of methylation capacity in the context of complex chronic illness, though the clinical utility of this application remains exploratory
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Yes
Collection method
Venipuncture at patient service centre
Turnaround
5–7 business days
Collection notes
Phlebotomy required. Fasting (8–12 hours, water permitted) is recommended to reduce postprandial methionine-load effects. Plasma must be separated from cells promptly — ideally within 30 minutes — and kept on ice during transport to prevent ex-vivo release from red cells. Confirm specimen-handling protocol with Cell Science Systems before ordering.
Specimen requirements
Serum or EDTA plasma; immediate centrifugation and separation required. If delayed processing is anticipated, collect in a pre-chilled tube on ice. Note specimen-handling compliance on the requisition.
What can affect Homocysteine results?
Homocysteine remethylation and transsulphuration depend on these vitamins; deficiency impairs clearance and raises plasma concentration, making homocysteine a functional indicator of nutritional status.
The thermolabile variant reduces methylenetetrahydrofolate reductase activity, impairing folate-dependent remethylation; homozygotes typically have modestly higher homocysteine, particularly when folate status is suboptimal.
Renal clearance contributes to homocysteine elimination; declining glomerular filtration raises plasma homocysteine independently of vitamin status, confounding cardiovascular interpretation.
Methotrexate and some antiepileptics interfere with folate metabolism; nitrous oxide irreversibly inactivates B12. A complete medication history is essential to interpretation.
Red cells release homocysteine ex vivo; plasma must be separated promptly (ideally within 30 minutes) and kept on ice to prevent spurious elevation.
Homocysteine rises with age and is typically higher in men than premenopausal women; reference intervals should be age- and sex-stratified.
What do Homocysteine results mean?
Elevated
Mild elevation (15–30 µmol/L) most often reflects dietary insufficiency of folate, B12, or B6, the MTHFR C677T polymorphism, chronic kidney disease, or medication effect. Moderate elevation (30–100 µmol/L) warrants exclusion of severe B12 deficiency, cobalamin transport defects, or CBS heterozygosity. Severe hyperhomocysteinaemia (>100 µmol/L) is diagnostic of homocystinuria (CBS or remethylation-pathway defects) and requires urgent metabolic evaluation and specialist referral.
Normal
A homocysteine concentration within the reference range does not exclude folate or B12 deficiency — serum and red-cell folate, methylmalonic acid, and holotranscobalamin offer complementary information. Normal homocysteine likewise does not rule out thrombophilia; other prothrombotic markers should be evaluated in the appropriate clinical context.
Reduced
Low homocysteine (<5 µmol/L) is uncommon and not clinically concerning; it may be seen with high-dose B-vitamin supplementation or in some genetic variants that enhance remethylation flux.
Note: Homocysteine must be interpreted in light of renal function, vitamin status, and medication history. Observational cardiovascular associations have not translated into benefit from B-vitamin intervention, so homocysteine-lowering for primary or secondary prevention is not guideline-endorsed. The specimen requires prompt separation to avoid artefactual elevation; note collection-handling protocol compliance.
Frequently asked questions
Homocysteine is an amino acid formed during methionine metabolism. Its plasma concentration reflects the balance between remethylation (folate- and B12-dependent) and transsulphuration (B6-dependent). It is measured to assess B-vitamin nutritional status, investigate unexplained thrombosis, and screen for homocystinuria.
How to access the Homocysteine test
The Homocysteine reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Refsum H, et al. Facts and recommendations about total homocysteine determinations: an expert opinion. Clin Chem. (2004)
- Marti-Carvajal AJ, et al. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane Database Syst Rev. (2017)
- Selhub J. Homocysteine metabolism. Annu Rev Nutr. (1999)
- American Heart Association Nutrition Committee. B vitamins and homocysteine — statement for healthcare professionals. (2006)
- Carmel R, Jacobsen DW, eds. Homocysteine in Health and Disease. Cambridge University Press. (2001)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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