AS diagnostic criteria: New York criteria revised in 1984: ① the duration of lower back pain lasts at least 3 months and the pain improves with activity but does not decrease with rest; ② the lumbar spine is limited in movement in the anterior-posterior and lateral flexion directions; ③ the thoracic extension is less than the normal value for the same age and sex; ④ bilateral sacroiliac arthritis grade II-IV, or unilateral sacroiliac arthritis grade III-IV. The diagnosis of AS can be confirmed if the patient has ④ and any one of ① to ③ respectively. New criteria for AS diagnosis in 2009: 1. With imaging results If the patient has imaging suggestive of sacroiliac arthritis and ≥1 SpA clinical features, the patient can be diagnosed as AS. 2. Without imaging results If the patient is HLA-B27 positive and has ≥2 other SpA clinical features, the patient can also be diagnosed as AS. Imaging suggestive of sacroiliac arthritis: ① MRI of the sacroiliac joint suggests active (acute) inflammation (definite bone marrow edema or osteitis), highly suggestive of sacroiliac arthritis associated with SpA; or ② X-ray suggestive of sacroiliac arthritis (same as the New York criteria revised in 1984). Clinical features of SpA: ① inflammatory back pain (IBP); ② arthritis; ③ tendonitis (heel); ④ uveitis; ⑤ finger (toe) inflammation; ⑥ psoriasis; ⑦ Crohn’s disease/ulcerative colitis; ⑧ good response to NSAIDs treatment (complete disappearance or significant improvement of pain 24-48 hours after drug administration); ⑨ family history of SpA (referring to one or two generations of relatives with AS, psoriasis, acute (either one of uveitis, reactive arthritis, inflammatory bowel disease); ⑩ HLA-B27 positive; ⑪ CRP elevated.