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Background Ankylosing spondylitis (AS) causes severe chronic inflammation of the spine and sacroiliac joints, leading to severe physical dysfunctions. Non-radiographic axial spondyloarthritis (nr-ax SpA) is a newly categorized disease in SpA that shows SpA without definite radiographic findings in sacroiliac joint. Although human leukocyte antigen (HLA) positivity is related to these diseases, little is known about the prevalence of these diseases and HLA-B27 positivity in Japan. Methods A nationwide survey was conducted from January to December 2017. 2221/8456 facilities (26.3%) were selected randomly as a target sample, comprising all three departments: orthopedics, pediatrics, and rheumatology. We estimated the number of these patients by using response and extraction rate, and calculated the HLA-B27 positivity rate. Results We estimated the prevalence of AS and nr-ax SpA as 2.6/100,000 (0.0026%) and 0.6/100,000 (0.0006%), respectively. HLA-B27 test was performed in 60% of patients with AS, of which 55.5% being HLA-B27 positive; however, they were less likely to receive HLA-B27 test if their estimated age of onset was over 50 years. Conclusions The prevalence in AS and nr-ax SpA and their HLA-B27 positivity rate in Japan were lower compared to other countries. Further studies will be required to reveal the association between HLA-B27 and the clinical features. Key messages The prevalence of AS and nr-ax SpA in Japan are estimated to be 2.6/100,000 (0.0026%) and 0.6/100,000 (0.0006%), respectively, and are lower than those in other countries. In addition, HLA-B27 positivity rate was lower.

1267Prevalence and HLA-B27 Positivity Rate among Patients with Ankylosing Spondylitis/Non-Radiographic Axial Spondyloarthritis in Japan