Occurrence of systemic lupus erythematosus in a Danish community: an 8-year prospective study

Objectives: To determine the prevalence and annual incidence of definite systemic lupus erythematosus (D-SLE) and incomplete SLE (I-SLE) in a community-based lupus cohort of predominantly Nordic ancestry in an 8-year prospective study from 1995 to 2003, and also to calculate the annual transition rate of I-SLE to D-SLE. Methods: In 1995 all SLE patients in the county of Funen were retrieved from four separate and independent sources. Incident cases were subsequently identified by surveillance of these sources. Results: During the 8-year study period the median annual incidence of D-SLE (1.04 per 100 000) and I-SLE (0.36 per 100 000) remained almost constant. The point prevalence (PP) of D-SLE increased from 21.9 to 28.3 per 100 000, and from 6.19 to 7.53 per 100 000 for I-SLE. During follow-up, seven I-SLE patients transformed into D-SLE at a progression rate of 3.64 per 100 person-years at risk [95% confidence interval (CI) 1.44-7.55]. Conclusions: Denmark is a low-incidence lupus area but lupus prevalence is increasing slowly. I-SLE is a disease variant that may eventually convert into D-SLE.