Making the Diagnosis
Your doctor will make the diagnosis of AS based on your symptoms and an X-ray or another type of imaging of your affected joints. The earliest sign is sacroiliitis (inflammation of the sacroiliac joint). It can be seen on an X-ray of the pelvis, or it can be detected at an even earlier stage with an MRI scan.
If you have AS, the X-ray will show areas where the bone has been worn away by the condition. The vertebrae of the spine may start to fuse together because the ligaments between them become calcified. The term for bones growing together due to inflammation is ankylosis, and this is where the name ankylosing spondylitis comes from. ("Spondyl" refers to the spine and "itis" means inflammation.) A physical exam includes the Schober test to assess the flexibility of the spine, which can be abnormal even when it's not obvious to the person.
Your doctor may also do a blood test called erythrocyte sedimentation rate (ESR) or a C-reactive protein test (CRP). A high ESR or CRP is a sign of conditions with inflammation, such as AS. However, it does not definitely mean that you have AS, since many other conditions can also cause a high ESR or CRP, and two-thirds of people with AS have a normal ESR.
Since back pain and osteoarthritis are so common and AS comes on so gradually, there is often a long delay in recognizing it, especially among doctors who are not specialized in rheumatology (the branch of medicine relating to the study and treatment of arthritis and other diseases that affect the joints, muscles, and bones).