Homocysteine

Overview

Homocysteine measures the sulfur-containing amino acid formed during the metabolism of methionine, which is then recycled to methionine by vitamin B12‑dependent methionine synthase or converted to cysteine by vitamin B6‑dependent enzymes. Chronically elevated levels (hyperhomocysteinemia) reflect impaired one‑carbon metabolism, often due to B‑vitamin deficiency (folate, B12, B6), genetic variants (e.g., MTHFR), or renal dysfunction. High levels are associated with increased risk of atherosclerotic cardiovascular disease, thrombosis, and, in severe cases, homocystinuria with multisystem complications. Clinically, homocysteine is used as a supplemental biomarker of folate/B12 status and cardiovascular risk, but direct causal roles in vascular disease remain debated.

Clinical Use Cases

  • Evaluating vitamin B12, folate, or vitamin B6 deficiency when anemia or macrocytosis is present.
  • Assessing cardiovascular risk in selected patients with borderline or multifactorial risk profiles.
  • Screening for suspected inborn errors of metabolism (e.g., cystathionine beta‑synthase deficiency, MTHFR‑related disorders).
  • Monitoring response to B‑vitamin and folate supplementation in hyperhomocysteinemia.

Specimen Types

  • Plasma (EDTA, with rapid processing and cold storage to prevent in vitro oxidation).
  • Serum (less preferred; levels may gradually rise after clotting unless separated promptly).

Measurement Methods

  • High‑performance liquid chromatography (HPLC) with fluorometric or electrochemical detection.
  • Immunochemiluminescent or enzymatic immunoassay methods.
  • Liquid chromatography–tandem mass spectrometry (LC‑MS/MS) in specialized laboratories.

Test Preparation and Influencing Factors

  • Fasting is often recommended, as methionine‑rich meals transiently raise homocysteine.
  • Certain medications increase levels (e.g., methotrexate, nitrous oxide, antiepileptics such as phenytoin, niacin).
  • Renal impairment, hypothyroidism, and aging tend to raise baseline homocysteine.
  • Folate, vitamin B12, and vitamin B6 supplementation can lower levels; testing should ideally precede or be coordinated with supplementation.

Synonyms

  • Total homocysteine.
  • Homocysteine, plasma.
  • Homocystine (when oxidized/disulfide form; often used in older literature).

Further Reading

  • “Homocysteine Test” – MedlinePlus; U.S. National Library of Medicine;