Understanding Your Test Results
Unfortunately, there is no single test to diagnose ankylosing spondylitis (AS). If your doctor suspects you may have AS he or she will order a combination of blood work and imaging tests, conduct a physical exam, and review your medical history. These tests include:
HLA-B27 antigen test
HLA-B27 is a genetic test to see whether you have the protein human leukocyte antigen B27 (HLA-B27) on the surface of your cells. If you test positive for HLA-B27 then you are likely to have one of the autoimmune disorders associated with the presence of HLA-B27. Ankylosing spondylitis is one of those conditions, along with juvenile rheumatoid arthritis, reactive arthritis, uveitis, and inflammatory bowel disease.
A positive HLA-B27 blood test does not mean you have AS, but that you are at increased risk for one of the conditions commonly seen in those with positive test results.
A negative HLA-B27 test means that this genetic marker was not found. However, you can still have AS without the HLA-B27 antigen.1
C-reactive protein (CRP)
This blood test checks for the presence of inflammation in your system. C-reactive protein (CRP) is made by the liver and released into the blood due to inflammation, injury or infection. High levels of CRP may point to an active or uncontrolled autoimmune disorder, and rising levels often precede pain or fever.1
Erythrocyte sedimentation rate (ESR)
This is another blood test that gives your doctor an idea of the inflammation levels in your body. An ESR test measures the rate of fall (sedimentation) of red blood cells (erythrocytes) in a test tube. It is estimated that less than 70 percent of people with AS have a raised ESR level.2
Complete blood count (CBC)
This blood test gives your doctor a sort of overview of all of how your body is operating. A CBC also checks for anemia, which can be a complication caused by the chronic inflammation of AS. The CBC will also show whether you have an increased number of white blood cells, which is another marker of inflammation.
Keep in mind: Only 30-40 percent of people with AS show inflammation in their CRP, ESR and CBC test results.3
X-ray & MRI
The hallmark sign of AS is inflammation or fusion of the vertebrae where the spine connects to the hips, also known as the sacroiliac joints. Inflammation in this area is called sacroiliitis.
The problem with using an X-ray to diagnose AS is that it can take 7 to 10 years for signs of spinal damage to become visible on X-ray (radiograph).2
MRI, or magnetic resonance imaging, may reveal changes in the spine or sacroiliac joints at an earlier stage of the disease.3
Because MRI is more sensitive than X-ray, it is considered better at detecting early-stage inflammation or swelling in the ligaments and other tissues of the sacroiliac joint that precede the structural changes later seen on X-ray.4 And even if you have no evidence of changes in your spine or hips, you may still have spondyloarthritis. Read more about non-radiographic spondyloarthritis here.
Those with AS typically report lower back pain and stiffness that lasts at least 3 months and began between their teens and 30s. The way that pain manifests for men and women can be different, which can sometimes lead to women having a delay in diagnosis. Men tend to have pain that is more concentrated at the base of the spine.1,2
Your doctor will also want to know if you have other symptoms of AS, such as iritis or uveitis, a history of gastrointestinal problems including Crohn’s or IBD, or a family history of AS.