Incidence of rheumatoid arthritis from 1995 to 2001: impact of ascertainment from multiple sources

The aim of this study was to describe the mean incidence rate of rheumatoid arthritis over a 7-year period from 1995 to 2001 in a population in the southern part of Denmark, using the data from several sources. Cases fulfilling the 1987 American College of Rheumatology criteria for rheumatoid arthritis were identified at hospitals and private practising rheumatologists (referral centres), and in general practice. The observed incidence was 32/100,000 person-years (95% confidence interval 29-35). Using the ratio between the number of cases known only from general practice and the number known from general practice and referral centres, the estimated incidence was 35/100,000 person-years (95% confidence interval 32-38). We suggest that the estimated rate should be viewed as a plausible upper limit for the incidence of rheumatoid arthritis in the southern part of Denmark.