A blood sample drawn from a vein in your arm
HLA-B27 is a specific protein (termed a human leukocyte antigen or HLA) that is found on cell surfaces. The term HLA-B27 is also used to refer to the gene that codes for the HLA-B27 protein. The HLA-B27 test determines the presence or absence of HLA-B27 protein on the surface of a person's white blood cells.
Human leukocyte antigens are proteins that help the body's immune system identify its own cells and distinguish between "self" and "nonself." Everyone has an inherited combination of HLA genes that code for the many antigens present on the surfaces of their cells. The presence or absence of each antigen creates a distinctive HLA combination for each person.
HLA-B27 is found in about 6% of the U.S. population. People with HLA-B27 have an increased likelihood of developing autoimmune diseases such as ankylosing spondylitis (AS), juvenile rheumatoid arthritis (JRA), reactive arthritis (of which one subset is Reiter syndrome), and isolated acute anterior uveitis. HLA-B27 has also been linked with inflammatory bowel disease and a range of other chronic conditions.
Ankylosing spondylitis and reactive arthritis are both chronic, progressive conditions that occur more frequently in men than women. The first symptoms typically occur when a person is in their early 30's. Often, the initial symptoms of these autoimmune disorders are subtle and may take several years before characteristic degenerative changes to bones and joints are visible on X-rays.
- Ankylosing spondylitis is characterized by pain, inflammation, and a gradual stiffening of the spine, neck and chest.
- Reactive arthritis is a group of symptoms that includes inflammation of the joints, urethra, and eyes as well as skin lesions.
- Juvenile rheumatoid arthritis is a form of arthritis that occurs in children.
- Anterior uveitis is associated with recurring inflammation of the structures of one or both eyes.
While HLA-B27 has not been established as a cause of any of these disorders, there is a higher prevalence of the antigen in those affected. For example, even though only 6% of the population has HLA-B27, 80-90% of people with ankylosing spondylitis are positive for HLA-B27.
How is it used?
The HLA-B27 test is primarily ordered to help strengthen or confirm a suspected diagnosis of ankylosing spondylitis (AS), reactive arthritis, juvenile rheumatoid arthritis (JRA), or sometimes anterior uveitis. The HLA-B27 test is not diagnostic; that is, it is not a definitive test that can be used to diagnose or rule out a disorder. The result adds information and is one piece of evidence used along with the evaluation of signs, symptoms, and other laboratory tests to support or rule out the diagnosis of certain autoimmune disorders, such as ankylosing spondylitis and reactive arthritis.
The HLA-B27 test may be ordered as part of a group of tests used to help diagnose and evaluate conditions causing arthritis-like chronic joint pain, stiffness, and inflammation. This group of tests may include a rheumatoid factor (RF) with either an erythrocyte sedimentation rate (ESR) or a C-reactive protein (CRP). HLA-B27 is sometimes ordered to help evaluate someone with recurrent uveitis that is not caused by a recognizable disease process.
When is it ordered?
An HLA-B27 test may be ordered when a person has acute or chronic pain and inflammation in the spine, neck, chest, eyes, and/or joints, and the healthcare practitioner suspects the cause is an autoimmune disorder that is associated with the presence of HLA-B27. An HLA-B27 test may also be ordered when someone has recurrent uveitis.
Healthcare practitioners frequently use the HLA-B27 test result when they suspect ankylosing spondylitis but the disease is in an early stage and the vertebrae in the spine have not yet undergone the characteristic changes that would be seen on X-ray.
What does the test result mean?
A positive HLA-B27 test means that the person tested is at increased risk of developing certain autoimmune diseases. If the person has symptoms such as chronic pain, inflammation, and/ or degenerative changes to bones (as seen on X-ray), then it supports a diagnosis of ankylosing spondylitis, reactive arthritis, or another autoimmune disorder that is associated with the presence of HLA-B27. This is especially true if the person is young, male, and if he experienced his first symptoms before the age of 40.
If an HLA-B27 test is negative, then the marker was not detected. This does not mean, however, that the person tested does not have the suspected condition since people who do not have the HLA-B27 antigen can also develop these autoimmune diseases. Likewise, someone who has the HLA-B27 antigen will not necessarily develop one of these conditions. Researchers are trying to determine what factors contribute to the higher likelihood of people with HLA-B27 developing these particular diseases and what actually triggers them.
Whether or not certain HLA antigens will be present is genetically determined. Genes that are passed from parents to their children control the production of those antigens. If two members of the same family are HLA-B27 positive and one of them develops a disease associated with HLA-B27, then the other person is at an increased risk of developing a similar disease.
Is there anything else I should know?
With new genetic testing methods, it is now possible to separate HLA-B27 into subtypes. So far, at least 105 different subtypes have been identified, such as HLA B27*05 and HLA B27*02. How the presence of these specific subtypes affects the likelihood of developing an autoimmune disease is not yet known.
What causes the autoimmune disorders associated with HLA-B27?
In most cases, the cause is not known. However, in some cases of reactive arthritis, there is an association between a previous infection by a microbe, such as Chlamydia, Campylobacter, Salmonella, Ureaplasma, or Yersinia, and the onset of the disease. It is thought that similarities between the HLA-B27 antigen and the antigens found on the surface of the microbes trigger the immune system to fight both the microbe and the person's own tissues, launching the autoimmune disorder after the resolution of the infection.
How long will it take for HLA-B27 results?
That depends on the laboratory performing the testing. HLA-B27 testing requires specialized equipment and expertise to perform. It is not offered by every laboratory. In most cases, your blood sample will be sent to a reference laboratory and may take a few days for results to be available.
Should everyone have an HLA-B27 test done?
Routine testing of the general public is not recommended. Usually only those with symptoms will be tested. A positive HLA-B27 in a person who does not have symptoms or a family history of HLA-B27-associated disease is not clinically significant. It does not help predict the likelihood of developing an autoimmune disease. Occasionally, a family member of a person who is positive for HLA-B27 and has an autoimmune disorder may be tested, but the test result cannot be used to predict whether the tested person will develop a related autoimmune disease.