In diagnosing ankylosing spondylitis (AS), a physician is likely to begin by reviewing the patient's medical history and performing a physical examination. X-rays may be taken to see whether or not the sacroiliac joint is inflamed. In severe and longstanding cases of AS, calcification of ligaments in the back occurs. On x-rays, this gives the back a "bamboo-spine" appearance. In some other cases, a bone scan or CAT scan (computed axial tomography) is used to view these internal features.

The physician may take a blood sample to check for the presence of genes such as HLA-B27, which is present in about 90 percent of patients with ankylosing spondylitis. Blood tests may also be used to check for elevated sedimentation rate - a measurement of the speed at which red blood cells settle to the bottom of a tube of blood in one hour - which indicates inflammation.

Blood testing can also reveal the presence of anemia, a condition in which there is a lack of healthy red blood cells to carry adequate oxygen to tissues. The chronic inflammation of AS may cause anemia.

Other tests may be ordered if a patient has symptoms suggesting a disorder related to AS, such as a colonoscopy for inflammatory bowel disease.