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. 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.)
Your doctor may also do a blood test called erythrocyte sedimentation rate, or ESR for short, 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.