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Also known as: Plasma total homocysteine
Formal name: Homocysteine

At a Glance

Why Get Tested?

To determine if you are folate-deficient or vitamin B12-deficient; to help diagnose a rare inherited disorder called homocystinuria; to determine if you are at increased risk of a heart attack or stroke

When to Get Tested?

When a doctor suspects a vitamin B12 or folate deficiency or suspects that an infant or young person may have homocystinuria; when you have had a heart attack or stroke and do not have traditional risk factors, as part of a cardiac risk assessment

Sample Required?

A blood sample taken by needle from a vein in the arm

Test Preparation Needed?

You may be instructed to fast for 10 to 12 hours prior to this test.

The Test Sample

What is being tested?

This test determines the level of homocysteine in the blood. Homocysteine is a sulphur-containing amino acid that is normally present in very small amounts in all cells of the body. Homocysteine is a product of methionine metabolism, and methionine is one of the eleven ‘essential’ amino acids (amino acids that must be derived from the diet since the body cannot produce them). In healthy cells, homocysteine is quickly converted to other products.

Folic acid (folate) is one of the ‘B’ vitamins that is needed to metabolise homocysteine. Vitamin B12, another B vitamin, helps keep folate in its active form, allowing it to keep homocysteine levels low. Therefore, people who are deficient in these vitamins may have increased levels of homocysteine.

According to the National Heart Foundation and the American Heart Association (AHA), some evidence suggests that excess homocysteine may promote atherosclerosis by damaging blood vessel walls and supporting the formation of inappropriate blood clots, but there is not a direct link between the two. The benefit of using homocysteine levels for risk assessment of cardiovascular disease (CVD), peripheral vascular disease and stroke is uncertain given that several studies indicate no benefit or lowering of CVD risk with folic acid and B vitamin supplementation.

Homocysteine can be greatly increased in the blood and urine of people with a rare inherited condition called homocystinuria. This disorder is caused by an alteration in one of several different genes. The affected person has a dysfunctional enzyme that does not allow the normal breakdown of methionine. Because of this, homocysteine and methionine begin to build up in the person's body. A baby with this condition will appear normal at birth but within a few years will begin to develop signs such as a dislocated lens in the eye, a long slender build, long thin fingers, skeletal abnormalities, osteoporosis and a greatly increased risk of thromboembolism and of atherosclerosis that can lead to premature cardiovascular disease. The build-up may also cause progressive mental retardation, behavioural disorders and seizures.

How is the sample collected for testing?

A blood sample is taken by needle from a vein in your arm.

Is any test preparation needed to ensure the quality of the sample?

Fasting for 10 to 12 hours may be required prior to blood testing.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.