Homocysteine

Homocysteine is a sulphur containing amino acid formed from methionine. It is cleared by remethylation to methionine, which needs methionine synthase, vitamin B12 and folate (as 5-methyltetrahydrofolate, made by MTHFR), or by transsulphuration to cystathionine and cysteine, which needs cystathionine beta synthase (CBS) and vitamin B6. A block in either route raises plasma homocysteine. Total homocysteine (free, protein bound and oxidised forms) is measured by immunoassay, enzymatic assay or chromatography and reported in micromol/L.

Its main uses are the diagnosis of classical homocystinuria, adjudication of borderline vitamin B12 and folate status, and the assessment of unexplained vascular events at a young age. It is a modifiable marker, though lowering it has not shown reduction of cardiovascular events in large studies, so it is not a routine cardiovascular screening test.

Mild hyperhomocysteinaemia is found in 5 to 10% of the general population and is more frequent in older adults. Classical homocystinuria from CBS deficiency is rare, with a birth prevalence of about 1 in 200 000 to 335 000, though it is more frequent in some populations (for example 1 in 2 000 in Qatar).

Indications. Suspected vitamin B12 or folate deficiency when levels are borderline, suspected homocystinuria (ectopia lentis, marfanoid habitus, intellectual disability, early thromboembolism, osteoporosis), unexplained venous or arterial thrombosis in young adults, neural tube defect in the family or a previous pregnancy, unexplained megaloblastosis, and monitoring of treatment.

Specimen. A fasting sample of at least 8 hours is preferred. Collect into EDTA (or heparin) and place on ice, centrifuge within 1 hour. If centrifugation is delayed, the concentration rises by about 10% each hour because red cells release homocysteine. Serum separated after clotting within 1 hour is acceptable. Once separated, the sample is stable for days.

Reference range. 5 to 15 micromol/L in adults (higher in older adults, and lower in pregnancy and in young women).

Severity bands.

• Under 15 micromol/L: normal.
• 16 to 30 micromol/L: moderate hyperhomocysteinaemia, usually nutritional, renal or drug related.
• 31 to 100 micromol/L: intermediate, usually severe B12 or folate deficiency, kidney failure or heterozygous CBS variants.
• Over 100 micromol/L: severe, suggesting classical homocystinuria or combined remethylation disorders.

Causes of raised homocysteine.

• Nutritional: vitamin B12 deficiency (often 30 to 100 micromol/L), folate deficiency, vitamin B6 deficiency (mild, mainly after methionine loading).
• Renal and endocrine: chronic kidney disease (levels rise as eGFR falls, commonly 20 to 50 micromol/L in dialysis), hypothyroidism.
• Drugs: methotrexate, trimethoprim, phenytoin, carbamazepine, valproate, metformin, nitrous oxide (inactivates B12), levodopa, fibrates, niacin, cholestyramine, and ciclosporin.
• Lifestyle and demographic: age, male sex, smoking, high coffee and alcohol intake, and low physical activity.
• Diseases: psoriasis, leukaemia, inflammatory bowel disease and other malabsorption, pernicious anaemia, and cancer (as a marker).
• Genetic: CBS deficiency (classical homocystinuria), severe MTHFR deficiency, methionine synthase and cobalamin disorders (cblC, cblD, cblE, cblG). The common MTHFR C677T variant has a small effect (homozygotes about 2 to 3 micromol/L higher) and is not itself a disease or an indication for treatment.

Causes of low homocysteine. Pregnancy, oestrogen therapy, folate rich diet, penicillamine, N-acetylcysteine, cystathionine beta synthase overactivity in Down syndrome. Low values have no clinical use.

Classical homocystinuria (CBS deficiency). Homocysteine usually over 100 micromol/L, with high methionine and low cysteine. Features: ectopia lentis, myopia, marfanoid habitus, osteoporosis, intellectual disability, and arterial and venous thromboembolism, the leading cause of death. Pyridoxine responsive forms are identified by response to a test course of pyridoxine. Management combines pyridoxine, folate, betaine and a methionine restricted diet.

Differentiating B12 and folate deficiency. Both raise homocysteine, while methylmalonic acid is raised in B12 deficiency alone. A raised homocysteine with normal methylmalonic acid suggests folate deficiency or a non-vitamin cause.

Cardiovascular and thrombosis risk. Moderate hyperhomocysteinaemia is a marker of vascular risk, with an association with stroke and venous thrombosis, but no benefit of vitamin therapy has been shown in large studies, and it is not used for thrombophilia screening in routine practice.

Critical values and findings needing immediate action.

• Homocysteine over 100 micromol/L, particularly in a child or young adult.
• Raised homocysteine with thrombosis, stroke, ectopia lentis or neurological features.
• Raised homocysteine in a woman planning pregnancy or in early pregnancy with a previous neural tube defect.
• Raised homocysteine with neurological symptoms and nitrous oxide exposure.

Interferences and pitfalls.

• Delay in separating the sample, or no cooling, falsely raises the result.
• A non-fasting sample after a meal or a protein load raises it by 10 to 15%.
• Renal impairment raises homocysteine independent of vitamin status, and so the vitamin results are needed to interpret.
• Folic acid supplements lower homocysteine rapidly, and a normal result on supplements does not exclude B12 deficiency.
• Drugs that lower or raise homocysteine alter results and are recorded.
• Haemolysis, lipaemia and high bilirubin have little effect on most methods.
• A single mild rise is not repeated unless it affects management.
• MTHFR genotyping is not a useful test for thrombophilia and does not explain mild hyperhomocysteinaemia on its own.

Special groups.

• Neonates: severe elevation indicates cobalamin disorder or CBS deficiency, and newborn screening for homocystinuria detects methionine in some programmes.
• Children: failure to thrive, developmental delay, lens dislocation and thrombosis suggest inherited disorders.
• Pregnancy: values fall in the first and second trimesters, and a high value is associated with neural tube defects and pre-eclampsia.
• Older adults: values rise with age and with lower B12 absorption, and are a sensitive early indicator of B12 deficiency.
• Regions: nutritional B12 deficiency in vegetarian populations and folate deficiency with malabsorption are important causes.

Follow up tests. Vitamin B12 and holotranscobalamin, red cell folate, methylmalonic acid, vitamin B6, creatinine and eGFR, thyroid function tests, full blood count and film, plasma amino acids (methionine, cystathionine, cysteine), urine homocystine, CBS gene sequencing, MTHFR enzyme assay, cobalamin disorder panel, slit lamp examination, bone density, and imaging for thrombosis.