At a Glance
Why Get Tested?
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
A blood sample taken by needle from a vein in the arm; sometimes a urine sample in addition to the blood sample
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 or urine. Homocysteine is a sulfur-containing amino acid that is normally present in very small amounts in all cells of the body. Homocysteine is a product of methionine metabolism. 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.
Vitamins B6, B12, and folate are necessary to metabolize homocysteine. People who are deficient in these vitamins may have increased levels of homocysteine.
According to 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 buildup may also cause progressive mental retardation, behavioral disorders, and seizures.
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.
Ask a Laboratory Scientist
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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.
Sources Used in Current Review
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 541-543.
Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 434-435.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 574-577.
What Is Homocysteine? American Heart Association [On-line information]. Available online at http://www.americanheart.org/presenter.jhtml?identifier=535 through http://www.americanheart.org. Accessed May 2009.
(Updated 2008 July). Familydoctor.org. [On-line information] Available online at http://familydoctor.org/online/famdocen/home/articles/249.html through http://familydoctor.org. Accessed May 2009.
(2008 January). Homocystinuria. Genetics Home Reference [On-line information]. Available online at http://ghr.nlm.nih.gov/condition=homocystinuria through http://ghr.nlm.nih.gov. Accessed May 2009.
(© 2009). Homocysteine, Folic Acid and Cardiovascular Disease. American Heart Association [On-line information]. Available online at http://www.americanheart.org/presenter.jhtml?identifier=4677 through http://www.americanheart.org. Accessed May 2009.
Baloghova, J. et. al. (2006 December 6). Homocystinuria. emedicine [On-line information]. Available online at http://emedicine.medscape.com/article/1115062-overview through http://emedicine.medscap.com. Accessed May 2009.
(Updated 2008 December). Cardiovascular Disease (Non-traditional Risk Markers) - Risk Markers - CVD (Non-traditional). ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/CardiacDz/CVDRiskMarkerNontrad.html through http://www.arupconsult.com. Accessed May 2009.
National Academy of Clinical Biochemistry. Laboratory Medicine Practice Guidelines: Emerging Biomarkers for Primary Prevention of Cardiovascular Disease and Stroke (2009) Homocysteine and Cardiovascular Disease Risk. Pg. 51. PDF available for download at http://www.aacc.org/members/nacb/LMPG/OnlineGuide/PublishedGuidelines/risk/Documents/PublishedGuidelines.pdf through http://www.aacc.org.
Sources Used in Previous Review
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO. Pp 488-490.
Stewart. D. (2004 July 26, Updated). Homocystinuria. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/001199.htm.
Picker, J. and Levy, H. (2005 August 15, Updated). Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency. GeneReviews [On-line information]. Available online at http://www.genetests.org/query?dz=homocystinuria through http://www.genetests.org.
(2005 September 23) Homocystinuria. Genetics Home Reference, Homocystinuria [On-line information]. Available online at http://ghr.nlm.nih.gov/condition=homocystinuria through http://ghr.nlm.nih.gov.
Genetic Fact Sheets for Parents, Amino Acid Disorders. Expanded Newborn Screening with Tandem Mass Spectrometry Financial, Ethical, Legal, and Social Issues [On-line information]. Available online at http://www.newbornscreening.info/Parents/aminoaciddisorders/CBS.html through http://www.newbornscreening.info.
(2005 May 02, Reviewed). Genetic Fact Sheets for Professionals, Amino Acid Disorders. Expanded Newborn Screening with Tandem Mass Spectrometry Financial, Ethical, Legal, and Social Issues [On-line information]. Available online at http://www.newbornscreening.info/Pro/aminoaciddisorders/CBS.html through http://www.newbornscreening.info.
Homocysteine May Trigger Strokes. JS Online. Available online at http://www.jsonline.com/alive/ap/feb01/ap-stroke-amino-ac021601.asp through http://www.jsonline.com.
Homocysteine: An emerging age-related cardiovascular risk factor. Geriatrics. April 1999.
Donald W. Jacobsen, PhD, FAHA. Director, Laboratory for Homocysteine Research, Department of Cell Biology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH. (Fellow, American Heart Association; American Association for Clinical Chemistry member).