Homocysteine is a compound (amino acid) that is involved in the production of several compounds in the cell. The use of homocysteine requires the presence of vitamins B6, B12 and folic acid. Because it is used quickly, homocysteine is normally present in very low concentrations in cells. High homocysteine levels can therefore indicate a deficiency in one of these vitamins. This test provides the results in micromoles of homocysteine per litre of blood (µmol/L).
High levels of homocysteine are associated with a higher risk of cardiovascular disease and stroke possibly by damaging the walls of blood vessels and promoting abnormal clot formation. However, the risk increase due to homocysteine is less significant than that due to the usual factors of cholesterol, hypertension, diabetes and smoking. In adults with abnormal blood clot formation (thrombophlebitis, etc.), it is sometimes useful to determine whether a mutation in the MTHFR gene responsible for high levels of homocysteine may be the cause. In infants, very high levels of homocysteine in the blood and urine may suggest a rare genetic disease called homocystinuria. This condition is due to a mutation in the genes that control the metabolism of homocysteine.