The development of ankylosing spondylitis is indeed closely related to the HLA-B27 antigen. The rate of HLA-B27 antigen positivity in patients with ankylosing spondylitis is as high as 90-96%. However, HLA-B27 antigen positivity alone cannot be used as a “diagnostic” or “confirmatory” test. The HLA-B27 antigen is positive in about 4% to 8% of the general population, and about 80% of HLA-B27 antigen-positive patients do not develop ankylosing spondylitis, and about 10% of patients with ankylosing spondylitis are HLA-B27 antigen-negative. HLA-B27 antigen-negative patients cannot be excluded from ankylosing spondylitis. The diagnosis of ankylosing spondylitis cannot be made with a positive HLA-B27 antigen, but must be confirmed by combining clinical symptoms, signs and symptoms with X-rays and CT examinations of the sacroiliac joints. Although there is a strong correlation between the HLA-B27 antigen and ankylosing spondylitis, the patient should not blindly assume that a positive HLA-B27 antigen means that he or she has ankylosing spondylitis. Because the probability of ankylosing spondylitis in HLA-B27 antigen-positive individuals is about 10% to 20%, it would be wrong to blindly assume that HLA-B27 antigen-positive individuals will definitely have ankylosing spondylitis, though. This suggests to us that HLA-B27 antigen is an important factor in the development of ankylosing spondylitis, but not the only one. In suspected or atypical clinical cases, if the HLA-B27 antigen is positive, it can only suggest that the possibility of ankylosing spondylitis should be highly thought of, but it does not establish that one must have ankylosing spondylitis. The diagnosis can be further confirmed by combining the symptoms, signs, and other relevant tests of the patient.