Increasing incidence and shifting profile of idiopathic inflammatory rheumatic diseases in adults during this millennium

To explore the trends in the incidence of idiopathic inflammatory rheumatic diseases (IIRDs) after the turn of the millennium. From a nationwide register maintained by the Social Insurance Institution of Finland, we collected all adult patients with IIRDs granted a new special reimbursement for anti-rheumatic drugs between 2000 and 2014. Temporal trends in the incidences of various IIRDs were estimated in three 5-year intervals. A total of 58,405 adult patients were identified. Between 2000–2004 and 2010–2014, the age-adjusted incidence rate of IIRDs increased from 114 to 116/100000 [incidence rate ratio (IRR) 1.03 (95% CI 1.01 to 1.06)] in women and from 67 to 69/100,000 [IRR 1.10 (95% CI 1.06–1.14)] in men. The incidence of seropositive rheumatoid arthritis (RA) remained stable while that of seronegative RA decreased. For other diagnoses, the incidences either increased (unspecified arthritis, psoriatic arthritis, spondyloarthritis), remained stable (reactive arthritis), or decreased (SLE and the group of diseases with the ICD-10 code M35). The gender difference in spondyloarthritis leveled as the incidence in women increased at a higher rate than in men. Mean age at IIRD diagnosis decreased among women. The total age-adjusted incidence of IIRDs has gradually increased, due to the increase in unspecified arthritis, psoriatic arthritis, and spondyloarthritis. This, in addition to the ascending number of individuals at risk in the population, translates into a growing burden to the health care system.