Ankylosing spondylitis (AS) is a condition that causes inflammation in the spine and other joints of the body, leading to symptoms of pain and stiffness. There is no single specific test to diagnose AS, and many of the symptoms (such as back pain) also can be caused by other conditions. A diagnosis often involves checking for common hallmarks of AS.
Medical history and physical exam
A doctor will assess the physical symptoms and the patient’s medical history to determine their risk of developing AS. For example, if previous family members have back problems or arthritis, this may increase the risk of a patient having AS. Previous or existing conditions, such as inflammatory bowel syndrome, skin rashes such as psoriasis, or eye problems, may indicate to a doctor that a patient’s symptoms are due to AS.
The doctor will ask questions about the symptoms of back pain. This questions include where and when the pain tends to be worse (such as in the morning or after a period of inactivity), if exercise relieves the pain slightly, and how long the pain has persisted.
A physical exam can test if the symptoms are consistent with AS. This includes identifying sites of pain and inflammation, testing the range of motion of the spine, and testing chest expansion by deep breathing.
Imaging tests
Imaging tests, such as an X-ray or magnetic resonance imaging (MRI) can produce images of the bones and joints often affected in AS. This can reveal evidence of ankylosis (fusing of parts of the spine) or sacroiliitis (inflammation of the site where the spine connects to the pelvis), both common symptoms of AS.
An early diagnosis can be difficult as the disease may not have progressed to the point where the damage is visible by X-rays. With MRI, it may be possible to identify the damage earlier, as it can visualize soft tissues.
If a patient has lower back pain lasting at least three months that gets better with exercise, but not with resting, limited movement in their lower back, and limited chest expansion for their age and sex, but no sacroiliitis, or have sacroiliitis only without the other three symptoms, they may be diagnosed with “probable AS.”
Laboratory tests
Laboratory testing, including blood tests, may be ordered to support a diagnosis of AS. A test for the HLA-B27 gene will be done, as this is often seen in patients with AS, especially in Caucasians. Blood may be tested for common indicators of inflammation, such as the presence of C-reactive protein (CRP), the erythrocyte sedimentation rate (ESR), and the plasma viscosity (PV) test. However, only about 30-40 percent of patients with AS will have a positive result in these tests, so this is only a guide.
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