The Test Sample
What is being tested?
Homocysteine is an amino acid that is typically present in very small amounts in all cells of the body. That is because the body normally converts homocysteine into other products quickly. Since vitamins B6, B12, and folate are necessary to metabolize homocysteine, increased levels of the amino acid may be a sign of deficiency in those vitamins. This test determines the level of homocysteine in the blood and/or urine.
Elevated homocysteine may also be related to a higher risk for coronary heart disease, stroke, peripheral vascular disease (fatty deposits in peripheral arteries), and hardening of the arteries (atherosclerosis). Several mechanisms have been proposed for how homocysteine leads to cardiovascular disease (CVD) risk, including damaging blood vessel walls and supporting the formation of inappropriate blood clots, but direct links haven not been confirmed. There are also several studies that indicate no benefit or lowering of CVD risk with folic acid and B vitamin supplements. So far, the American Heart Association does not consider it a major risk factor for heart disease.
A rare inherited condition called homocystinuria (also called cystathionine beta synthase deficiency) can also greatly increase homocysteine in the blood and urine. In homocystinuria, one of several different genes is altered, leading to a dysfunctional enzyme that does not allow the normal breakdown of the precursor to homocysteine, called methionine. Methionine is one of the eleven essential amino acids that must come from the diet because the body cannot produce it.
Without the proper enzyme to break them down, homocysteine and methionine begin to build up in the body. Babies with this condition will appear normal at birth but within a few years 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 CVD.
The buildup in the arteries may also cause intellectual disability, mental illness, slightly low IQ, behavioral disorders, and seizures. Some of those may be alleviated if the condition is detected early, which is why all states screen newborns for homocystinuria.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm. Sometimes a urine sample is also collected.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
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.