HLA-B27 antigenDefinition: HLA-B27 is a blood test to look for� human leukocyte antigen B27 (HLA-B27) on the surface of� lymphocytes (white blood cells). Human leukocyte antigens (HLA) are proteins that help the body�s defense system tell the difference between its own cells and other cells. HLA type B27 is associated with autoimmune diseases such as� ankylosing spondylitis. See also: Histocompatibility antigens test.
Alternative Names: Human leukocyte antigen B27
How the test is performed: Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. In infants or young children: The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.
How to prepare for the test: No special preparation is usually necessary.
How the test will feel: When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed: The human leukocyte antigens (HLAs) are proteins present on the surface of all body cells that contain a nucleus, and are in especially high concentrations in white blood cells (leukocytes). HLA antigens are the major histocompatibility antigens for tissue recognition. They are especially important in considering any type of tissue transplant, for example, kidney transplant or bone marrow transplant. Many HLA antigens exist, but some are of special interest since they are more common in certain autoimmune diseases. For example, HLA-B27 is found in 80 - 90% of people with ankylosing spondylitis and Reiter's syndrome. However, HLA-B27 is also present in 5 - 7% of Caucasian people without autoimmune disease. Using the new genetic testing methods, not all HLA-B27 genes are associated with an increased risk of developing ankylosing spondylitis (for example, HLA-B*2706 has not been shown to have an association). It is important to note that this test does not predict the development of autoimmune disease, and should not be used for this purpose.
Normal Values: HLA-B27 is found in 5 - 7% of Caucasian people without autoimmune disease.
What abnormal results mean: A positive test indicates a greater-than-average risk for developing: - Ankylosing spondylitis
- Reiter's syndrome
- Sacroiliitis (inflammation of the sacroiliac joint)
In the presence of suggestive clinical findings, a positive HLA-B27 test may confirm the diagnosis.
What the risks are: - Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
Special considerations: Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
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