Incidence of Rheumatoid Arthritis in Sweden: A Nationwide Population-Based Assessment of Incidence, Its Determinants, and Treatment Penetration

Objective To estimate the nationwide incidence of rheumatoid arthritis (RA) in Sweden, including its variation across age, sex, geography, and demography, and to describe the sensitivity of register-based incidence estimates to different RA case definitions. Methods Incident RA patients were identified using the Swedish National Patient Register. In the base case, incident RA was defined as first-ever inpatient or nonprimary outpatient care visit listing an RA diagnosis in 2006-2008, with a second visit listing RA within 1 year. Patients prescribed disease-modifying antirheumatic drugs more than 6 months prior to the first visit listing RA were not regarded as incident. The robustness of this definition was evaluated by more liberal and strict criteria, and by penetration of antirheumatic treatment. Results Between 2006 and 2008, 8,826 individuals were identified as incident RA patients. The overall incidence was 41 per 100,000 (56 for women, 25 for men). The incidence increased with age and peaked in the 70-79 years age group for both women and men. The age- and sex-standardized incidences were lower in densely populated areas and in individuals with high educational level. No geographic trends were noted. More liberal and strict definitions of RA only altered the observed incidence by approximately 14%. Conclusion The overall nationwide register-based incidence of RA was robust across different case definitions. In a country with universal access to care, RA displayed demographic and socioeconomic, but no geographic, variations in incidence, and peaks at an older age than most commonly reported, with no difference in peak age at RA onset between sexes.