Diagnosing Ankylosing Spondylitis and the Role of HLA B-27 Testing

Frequently asked questions and answers about the Human Leukocyye Antigen B-27 blood test.

Diagnosing Ankylosing Spondylitis and the Role of HLA B-27 Testing

by Dr. C.H. Weaver M.D. updated 11/2018

The diagnosis of ankylosing spondylitis (AS) is based upon history, physical examination, and radiographic findings. Inflammatory back pain is characterized by the presence of back pain for longer than 3 months in association with an age of onset before 40 years, no improvement with rest, improvement with exercise, insidious onset, and pain increased at night. Plain x-ray films of the lower spine will show definite signs of arthritis in the sacroiliac joints. Laboratory findings of HLA-B27 positivity, and blood test signs of inflammation (elevated C reactive protein or erythrocyte sedimentation rate) are compatible but not specific for AS.

What is the HLA b27 gene?

Human leukocyte antigens (HLAs) are proteins commonly found on white blood cells. These antigens help the immune system identify the differences between healthy body tissue and foreign substances that may cause infection.

Although most HLAs protect the body from harm, HLA-B27 is a specific type of protein that contributes to immune system dysfunction. The presence of HLA-B27 on white blood cells can cause an individual’s immune system to attack normal healthy cells. This is referred to as an autoimmune disease.

What diseases are associated with the HLA b27 gene?

The presence of HLA-B27 is associated with certain autoimmune and immune-mediated diseases. In addition to its association with AS, HLA-B27 is implicated in other types of seronegative spondyloarthropathy as well, such as reactive arthritis (formerly known as Reiter's Syndrome), certain eye disorders such as acute anterior uveitis and iritis, psoriatic arthritis and ulcerative colitis. These autoimmune conditions cause inflammation of the joints, spine urethra, and eyes, and sometimes the skin.

What is an HLA-B27 test?

An HLA-B27 test is a blood test that identifies the HLA-B27 proteins located on the surface of white blood cells.

What does a positive HLA b27 test mean?

A positive test means that the HLA-B27 antigen was identified on the white blood cells and individuals that test positive have a higher-than-average risk of developing an autoimmune disease including ankylosing spondylitis.

How common is HLA b27?

HLA B-27 is present in about 6%-8% of individuals and approximately 1% of B27 positive individuals develop ankylosing spondylitis. HLA-B27 is more common among white individuals that other ethnic groups.

How is the HLA b27 gene inherited?

There is a 50% chance that the child of one HLA-B27 positive parent will inherit the gene, but a smaller chance of actually developing spondylitis. This ranges from about 5% to 20%.

How much does HLA B27 cost?

HLA-B27 Antigen test cost is between $109.00 and $219.00.

How are the results of the HLA B-27 test interpreted?

A negative test indicates the absence of HLA-B27 on an individual’s white blood cells. A negative test doesn’t mean than an individual does not have an autoimmune disorder. Sometimes people with autoimmune disorders don’t have HLA-B27 on their white blood cells.

A positive test simply means that the HLA-B27 antigen is present. The presence of the antigen doesn’t always mean that an autoimmune disorder will develop. Diagnosis of an autoimmune disorder must be made based on symptoms and the results of other blood tests and diagnostic exams.

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