The differential diagnosis of ankylosing spondylitis (a) lumbosacral joint strain Chronic lumbosacral joint strain is a persistent, diffuse lumbago, with the lumbosacral region being the most severe, with no restriction of spinal movement and no special changes on X-ray. In acute lumbosacral joint strain, the pain is aggravated by activity and can be relieved after rest. (B) Osteoarthritis Often occurs in the elderly, characterized by bone and cartilage degeneration, synovial thickening, more common in the weight-bearing spine and knee joints. Chronic low back pain is often the main symptom when the spine is involved, which is easily confused with AS; however, joint ankylosis and muscle atrophy do not occur in this product, and there are no systemic symptoms. (C) Forestier disease (age-related ankylosing bone hypertrophy) also occurs in the spine, similar to the bamboo-like changes in the spine of AS, but the sacroiliac joint is normal and the small intervertebral joints are not violated. (D) tuberculous spondylitis Clinical symptoms are similar to AS, but X-ray examination can be used to differentiate. In tuberculous spondylitis, the vertebral margins are blurred, the intervertebral space is narrowed, the anterior wedge is changed, there is no ligamentous calcification, sometimes there is a shadow of paravertebral tuberculous pustules, and the sacroiliac joints are unilaterally involved. (E) Rheumatoid arthritis It has been confirmed that AS is not a specific type of RA, and there are many differences to differentiate between the two. RA is more common in women, usually invades the small joints of the hands and feet first, and is bilaterally symmetrical, while the sacroiliac joints are usually not involved. (vi) Enteropathic arthropathy Ulcerative colitis, restrictive enteritis or enterogenic lipid metabolism disorder (Whipple) can occur in spondylitis, and enteropathic arthropathy involves joints and X-ray changes are similar to AS and not easily distinguished, so it is necessary to look for intestinal symptoms and signs for differentiation. Ulceration of the colonic mucosa, edema and bloody diarrhea in ulcerative colitis; abdominal pain, nutritional disorders and fistula formation in restrictive enterocolitis; steatorrhea and acute wasting in Whipple’s disease all contribute to the diagnosis of the primary disease. Enteropathic arthropathy has a low HLA-B27 positivity rate, and IgG is increased in intestinal perfusate in patients with Crohn’s disease, while IgG is essentially normal in intestinal perfusate in patients with AS. (vii) Reiter syndrome and psoriatic arthritis Spondylitis and sacroiliac arthritis can occur in both diseases, but spondylitis generally occurs later and is milder, with less calcification of paravertebral tissues and non-marginal type (calcification of fibrous tissues outside the fibrous ring) of ligamentous bone redundancy, forming partial bone bridges between two adjacent vertebrae different from the bamboo-like spine of AS; sacroiliac arthritis is generally unilateral or bilateral asymmetrical, with osteoarthritic joints lesions are rare and there is no generalized osteoporosis. In addition, Reiter syndrome has conjunctivitis, uveitis, and mucosal skin damage, and psoriatic arthritis has skin psoriasis damage for differentiation.