New York criteria revised in 1984: 1. Clinical criteria (1) low back pain and stiffness for more than 3 months, with improvement in activity and no improvement at rest (2) restricted movement in the frontal and sagittal planes of the lumbar spine (3) thoracic mobility lower than normal for the corresponding age and sex 2. Radiological criteria Bilateral sacroiliac arthritis ≥ grade 2 or unilateral sacroiliac arthritis grade 3-4 3. Diagnostic criteria (1) Definite ankylosing spondylitis: meeting radiological criteria and more than 1 clinical criteria (2) possible ankylosing spondylitis: ① meet 3 clinical criteria; ② meet radiological criteria without any clinical criteria (other causes of sacroiliac arthritis should be excluded) 2009: International Spondyloarthropathy (SpA) Evaluation Working Group (ASAS) published the Inflammatory Back Pain (IBP) criteria and the new mid-axis SpA criteria. Patients with low back pain lasting at least 3 months and an age of onset <45 years are diagnosed with mid-axis SpA (prototype AS) if any of the following criteria are met: 1. When imaging findings are available If the patient has imaging suggestive of sacroiliac arthritis with ≥1 clinical feature of SpA 2, without imaging results If the patient is HLA-B27 positive and has ≥2 other SpA clinical features. Note: The 5 criteria for IBP are: ① age of onset <40 years; ② insidious onset; ③ improvement after exercise; ④ failure to improve after rest; ⑤ nocturnal pain (improvement after waking up). If the patient met four of these criteria, the presence of IBP was determined (sensitivity 77.0%, specificity 91.7%). Imaging suggestive of sacroiliac arthritis: ① MRI of the sacroiliac joint suggested active (acute) inflammation (definite bone marrow edema or osteitis), highly suggestive of the presence 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 IBP; ② arthritis; ③ tendon telangiectasia (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 uveitis reactive arthritis, inflammatory bowel disease in any of the diseases); ⑩ HLA-B27 positive; (11) elevated CRP.