The diagnosis of AS is based upon a spectrum of characteristic findings noted in the history physical examination, and radiographic findings. Inflammatory back pain is characterized by the presence of back pain for longer than 3 months in association with an age of onset before 40 years, no improvement with rest, improvement with exercise, insidious onset, and pain increased at night. Plain x-ray films of the lower spine will show definite signs of arthritis in the sacroiliac joints. Laboratory findings of HLA-B27 positivity, and blood test signs of inflammation (elevated C reactive protein or erythrocyte sedimentation rate) are compatible but not specific for AS.
Inflammation of AS starts before X-ray findings is identified. This is thought of as a pre-radiographic form of AS. In these individuals with early AS, MR scan of the spine can identify inflammation in areas of the spine compatible with the diagnosis.