Objective New classification criteria for axial spondyloarthritis (SpA) have been validated by the Assessment of SpondyloArthritis international Society (ASAS) working group. We applied these criteria to estimate prevalence of SpA in randomly selected, retrospectively reviewed medical records from representative US rheumatology practices. Methods Rheumatologists from 101 US practices identified at-risk patients, ages 18-44 years, with chronic back pain. Medical records were reviewed against ASAS criteria. The proportion of patients meeting ASAS criteria was compared to an estimate of the total number of at-risk patients treated at participating sites and, following weighting, was extrapolated to 5,520 US rheumatology practices. US Census data were used to estimate national prevalence. Results In a sample of 816 randomly selected records, 514 (63%) at-risk patients (95% confidence interval [95% CI] 59.6-66.3%) met ASAS criteria. By applying this proportion to 1,217,097 Americans estimated at risk, 766,652 were projected to meet ASAS criteria. This projection corresponds to a national prevalence of 0.70% (95% CI 0.38-1.1%) or 701 per 100,000 individuals. The prevalence estimates of ankylosing spondylitis and nonradiographic axial SpA are 0.35% (95% CI 0.18-0.554%) and 0.35% (95% CI 0.18-0.554%), respectively. Rheumatologists diagnosed axial SpA in 491 (60%) of those at risk, corresponding to 0.67% (95% CI 0.36-1.01%) prevalence overall. However, of 514 patients meeting ASAS criteria, 124 (24%) were undiagnosed by rheumatologists. Conclusion This is the first systematic epidemiology study of axial SpA using ASAS criteria. Better recognition of axial symptoms is needed, as rheumatologists' expert clinical diagnoses are not always in agreement with ASAS criteria.
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