Total Plasma Homocysteine (tHCy)

Coagulation

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Description

Homocysteine (Hcy) is a non-protein forming sulphur containing amino acid. The source of homocysteine is methionine, derived from dietary protein. Hcy lies at the centre of two metabolic pathways - the remethylation pathway and the transsulphuration pathway, B vitamins are co-factors. In methionine excess Hcy is irreversibly metabolised to cysteine by the action of the enzyme cystathione b synthase (CBS) in the vitamin B6 dependent transsulphuration pathway.

In a state of negative methionine balance Hcy is remethylated to methionine in the main by the folate and cobalamin (vitamin B12) dependent enzymes methionine synthase (MS) and methylenetetrahydrofolate reductase (MTHFR). Total homocysteine (tHcy) represents the sum of all forms found in plasma. Assuming normal renal function, increased levels of plasma tHcy indicate that intracellular homocysteine metabolism is disrupted and that the export mechanism is disposing excess homocysteine into the blood.

Homocystinuria is associated with a very high incidence of both arterial and venous thrombosis, many subjects experiencing a vascular occlusive event by the age of 16 years. The cause of homocystinuria in 90% to 95% of cases, is homozygosity for deficiency of cystathione-â-synthase (CBS); similar high homocysteine levels may be found with homozygous MTHFR deficiency. Homozygous CBS deficiency has a prevalence of around 1 in 250 000 of the general population. Mild and moderate increase of tHCy level has been associated with some increased thrombotic risk but the assay is not indicated for routine thrombophilia screening.

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Indications

If a high level of homocysteine is suspected and raised level may influence treatment. To monitor tHCy level in patients with known congenital homocystinuria.

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Sample Type

Plasma (Citrate Blue) x 2 must be processed by lab within 1 hour of venepuncture.

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Reference Range

Gender specific reference ranges indicated on report.

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Turnaround Time

Within 2 weeks

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Testing Frequency

Weekly.

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External Notes

Separate male and female normal ranges are used and pregnancy considerably reduces tHCy level. Renal failure, HIV, cancer, severe psoriasis, hypothyroidism, pernicious anaemia and certain medications may increase homocysteine levels. Smoking, coffee, age and physical exercise have also been shown to increase plasma levels. Higher homocysteine levels have been reported in vegetarians, compared to non-vegetarians.

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Patient Preparation

Fresh citrated blood (2.7ml Blue). Falsely increased levels if the plasma is not removed from blood cells within 1 hour of venepuncture.Homocysteine in separated plasma is stable for 96 hours at room temperature, and can be sent to us by first-class post.

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Please note: the above information is subject to change and we endeavour to keep this website up to date wherever necessary.