How Homocysteine is Measured
Fasting blood tests are usually the manner in which homocysteine is measured. For standard reference purposes, total fasting plasma homocysteine is recommended. A patient who has had a serious illness, such as a myocardial infarction or cerebrovascular accident, should not be tested for 8-12 weeks after the event because there is a potential for the homocysteine level to be abnormal during that time period. The reason for this effect has not been elucidated. Before a blood sample is taken, the patient should be advised to fast for 10-12 hours. Between 0.6 and 1.2 mL of blood is collected in a green-top or purple-top tube for total homocysteine assay. The blood should be centrifuged within 30 minutes to 1 hour of collection, and must be refrigerated or frozen until the time of analysis. Some studies with negative results have been criticized for using plasma samples that were stored in liquid nitrogen for extended periods. Storing blood at room temperature may cause homocysteine levels to be falsely elevated due to the export of homocysteine from red blood cells or hemolysis. If the sample is sent to an outside laboratory for analysis, the serum should be shipped on dry ice, via overnight delivery. The cost of fasting homocysteine analysis ranges from $50-$110. It is advisable to check with individual insurance and managed care companies to ask if they will pay for the screening test.
Similar to a glucose tolerance test for metabolic abnormality, a methionine loading test can be performed to assess methionine metabolism. In some laboratories a granulated form of 100 mg/kg of methionine is dissolved in a glass of fruit juice and given orally after 10 hours of fasting. Blood samples are obtained before and at 4, 8, 12, and 24 hours after loading. In other laboratories, the post-loading sample is taken once at 6 hours and compared to the fasting value. The methionine loading test is more expensive; however, the fasting homocysteine test can be made more sensitive by lowering the normative range criterion. A methionine loading test is used by van der Griend et al. to diagnose hyperhomocysteinemia because a considerable number of people with high homocysteine would remain undiagnosed with the determination of a fasting homocysteine level alone. This test is not conducted on patients whose creatinine levels exceed 1.5 mg/dL, because elevated creatinine levels indicate malfunctioning kidneys that cannot effectively filter protein (in this case, methionine). Comparisons of serum folic acid, vitamins B6 and B12, and creatinine should be measured at the same time as homocysteine.
A total plasma homocysteine level of ≤12 µmol/L is considered optimal, but the algorithm used by Drs. McBride and Stein starts secondary prevention at ≥11 µmol/L and considers ≥14 µmol/L to require primary prevention. Those numbers vary somewhat, depending on which study is being followed. A concentration above 15 µmol/L is associated with a high risk of occlusive vascular disease. If the homocysteine level is ≥100 µmol/L, the patient should be referred to a geneticist, especially if several members of the same family have hyperhomocysteinemia.
Prog Cardiovasc Nurs. 2002;17(1) © 2002 Le Jacq Communications, Inc.
© 2007 Prog Cardiovasc Nurs
Cite this: Emergent Cardiovascular Risk Factor: Homocysteine - Medscape - Jan 01, 2002.