Incidence of systemic lupus erythematosus and lupus nephritis in Denmark: a nationwide cohort study

OBJECTIVE: To determine the incidence of systemic lupus erythematosus (SLE) and SLE with concomitant or subsequent lupus nephritis (LN) in Denmark during 1995-2011, using data from the Danish National Patient Registry (NPR).

METHODS: To assess the incidence of SLE, we identified all persons aged ≥ 18 years in the NPR with at least 1 International Classification of Diseases, 10th ed (ICD-10) code of SLE and at least 365 days of followup under this diagnosis. Identification of LN cases was based on fulfillment of these criteria and ≥ 1 registration under an ICD-10 code of nephritis concomitantly with or after first SLE registration.

RESULTS: The overall annual incidence rate per 100,000 for SLE was 2.35 (95% CI 2.24-2.49); 0.69 (95% CI 0.60-0.78) for men and 3.96 (95% CI 3.75-4.17) for women. For LN, the mean annual incidence rate per 100,000 was estimated to be 0.45 (95% CI 0.38-0.53); 0.20 (95% CI 0.13-0.28) for men and 0.69 (95% CI 0.57-0.83) for women. The differences in SLE incidence rates between sexes decreased by age, and the incidence did not differ between men and women after the age of 60 years for LN. The estimated incidences showed no trends by calendar time. Estimated overall point prevalence (December 31, 2011) per 100,000 was 45.2 (95% CI 43.3-47.4) and 6.4 (95% CI 5.7-7.2) for SLE and LN, respectively.

CONCLUSION: Our Danish population-based data showed a stable incidence of SLE and LN. As expected, we found higher incidence rates among women than among men, particularly in younger persons.