The most common characteristic early complaints of AS are morning stiffness and lower back pain. Low back pain is an extremely common symptom in the general population, but most are mechanical back pain non-inflammatory pain, whereas AS is inflammatory pain. The following 5 items help to differentiate inflammatory back pain caused by AS from non-inflammatory back pain caused by other causes: 1. back discomfort occurred before the age of 40; 2. slow onset; 3. symptoms lasted for at least 3 months; 4. back pain was accompanied by morning stiffness; 5. back discomfort decreased or disappeared after activity. If 4 of the above 5 items are met, inflammatory back pain is supported.