ABSTRACT
OBJECTIVE: To investigate and compare trends in incidence rates (IRs) of seropositive and seronegative rheumatoid arthritis (RA) in Denmark using various data sources for serostatus definition. METHOD: This nationwide population-based cohort study was based on data from Danish healthcare and clinical quality registries between 2000 and 2018. Information on anti-cyclic citrullinated peptide and immunoglobulin M rheumatoid factor was obtained, and definitions of seropositivity according to the number of applied data sources were prespecified. Annual age- and sex-standardized IRs were calculated as the number of incident seropositive and seronegative cases, divided by the number of person-years (PY) in the general population in that given year. RESULTS: An increasing temporal trend in IR of seropositive RA and a decreasing trend in seronegative RA were observed. The IRs were higher for seropositive RA than for seronegative RA from 2009 onwards, with a widening of the IR gap between 2009 and 2016 regardless of the definition of seropositivity. When combining laboratory- and physician-reported autoantibody information and ICD-10 codes, the IR of seropositive RA in 2018 was approximately twice that of seronegative RA, at 19.0 and 9.0 per 100 000 PY, respectively. The level of antibody testing increased significantly during the study period. CONCLUSIONS: The IR of seropositive RA increased over time, whereas the IR of seronegative RA decreased. Temporal IR changes may be caused by a real change in the RA serology subtypes, an increase in autoantibody testing and availability, changes in registration practice over time, or a combination of these factors.
ABSTRACT
OBJECTIVE: To investigate the incidence and prevalence of rheumatoid arthritis (RA) in the adult Danish population. METHOD: In this nationwide register-based cohort study, patients with incident RA between 1998 and the end of 2018 were identified using Danish administrative registries. The age- and sex-standardized incidence rate (IR), incidence proportion (IP), lifetime risk (LR), and point prevalence (PP) of RA were calculated. RA was defined as a first-time RA diagnosis registered in the Danish National Patient Registry combined with a redeemed prescription of a conventional synthetic disease-modifying anti-rheumatic drug in the following year. In addition, three different case definitions of RA were explored. RESULTS: The overall age- and sex-standardized IR of RA from 1998 to 2018 was 35.5 [95% confidence interval (CI) 35.1-35.9] per 100 000 person-years while the IP was 35.2 (95% CI 34.8-35.5) per 100 000 individuals. The IR was two-fold higher for women than for men. The LR of RA ranged from 2.3% to 3.4% for women and from 1.1% to 1.5% for men, depending on the RA case definition used. The overall PP of RA was 0.6% (95% CI 0.5-0.6%) in 2018: 0.8% (95% CI 0.7-0.8%) for women and 0.3% (95% CI 0.3-0.4%) for men. The prevalence increased about 1.5-fold from 2000 to 2018. CONCLUSION: The IR and PP were approximately two-fold higher for women than for men. The prevalence of RA in Denmark increased significantly from 2000 to 2018. The RA case definition had more impact on the results than the choice of denominator.
