Objective: We aimed to determine the population-based incidence, prevalence, and mortality of dermatomyositis (DM) using European Alliance of Associations for Rheumatology (EULAR)/American College of Rheumatology (ACR) criteria. Methods: This population-based cohort study included incident DM from January 1, 1995 to December 31, 2019. We manually reviewed all individuals with at least 1 code for DM or polymyositis to determine if they met EULAR/ACR criteria, subspecialty physician diagnosis, and/or Bohan and Peter criteria. We age- and sex-adjusted incidence and prevalence estimates to the US non-Hispanic White year 2000 population and estimated prevalence on January 1, 2015. Standardized mortality ratios (SMRs) with 95% confidence intervals (95% CIs) compared observed to expected mortality adjusting for age, sex, and year. Results: We identified 40 cases of verified DM, with 29 cases incident in Olmsted County from 1995 to 2019. The mean age was 57 years, 26 (90%) were female, and 12 (41%) had clinically amyopathic DM (CADM). The median follow-up time was 8.2 years. The overall adjusted incidence of DM was 1.1 (95% CI 0.7–1.5) per 100,000 person-years, and prevalence was 13 (95% CI 6–19) per 100,000. The SMR was significantly elevated among the myopathic DM cases (3.1 [95% CI 1.1–6.8]) but not CADM cases (1.1 [95% CI 0.2–3.3]). The positive predictive value of ≥2 DM codes was only 40 of 82 (49%). Conclusion: This population-based study found that DM incidence and prevalence were higher than previously reported. Mortality was significantly elevated for myopathic DM but not for CADM.
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