HLA-B27 is a blood test to look for specific protein found on the surface of white blood cells. The protein is called human leukocyte antigen B27 (HLA-B27).
Human leukocyte antigens (HLAs) are proteins that help the body’s immune system tell the difference between its own cells and foreign, harmful substances.
See also: Histocompatibility antigen test
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.
A normal (negative) result means HLA-B27 is absent.
However, HLA-B27 is found in 5 - 7% of Caucasians who do not have an autoimmune disorder.
What abnormal results mean:
A positive test means HLA-B27 is present. It suggests a greater-than-average risk for developing:
- Ankylosing spondylitis
- Reiter 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:
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.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)