OBJECTIVE: To determine the concordance of systemic lupus erythematosus (SLE) and co-aggregating autoimmune diseases among Danish twins.

METHODS: SLE-affected twins were ascertained by record linkage between the National Patient Register (NPR) and the Danish Twin Registry (DTR). Registered SLE codes were validated through medical chart review and information from the treating physicians. Twin pairs with at least one chart-validated SLE proband were invited to participate in a personal interview and clinical validation of the SLE diagnoses.

RESULTS: Twenty-two twins fulfilled the ACR criteria for SLE. The age- and sex-adjusted point SLE prevalence in the Danish twin cohort was 30.3 per 100,000 persons (95% CI: 19.2-46.5). Among seven monozygotic (MZ), eight same-sex dizygotic (DZss) and five opposite-sex dizygotic (DZos) twin pairs, one MZ and one DZss were concordant for SLE. This corresponded to probandwise concordance rates of 25.0% (95% CI: 7.15-59.1) and 7.7% (95% CI: 1.37-33.3), and pairwise concordance rates of 14.3% (95% CI: 2.57-51.3) and 7.7% (95% CI: 1.37-33.3) among MZ and DZ twins, respectively. An SLE diagnosis was clinically validated in 17 twins from 15 twin pairs. Another four co-twins had other autoimmune disease, corresponding to a probandwise concordance of any autoimmune disease of 50.0% in MZ (95% CI: 21.5-78.5) and 23.1% in DZ twins (95% CI: 8.18-50.3).

CONCLUSION: Population-based Danish data suggest that SLE twin concordance is lower than previously reported, but still point to the importance of both genetic and environmental factors, and indicate a substantial co-aggregation of other autoimmune diseases in SLE twins.