Abstract
OBJECTIVE: To determine frequency, demographic and treatment characteristics of patients with an overlapping second autoimmune illness (2nd AI). METHODS: We analysed two cohorts containing 897 patients with 'pure' systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren's syndrome or antiphospholipid syndrome (APS) and 424 patients with one of these diagnoses plus at least one 2nd AI. RESULTS: A 2nd AI occurred in 38% of all patients diagnosed as having SLE (with or without a 2nd AI), 30% with RA, 52% with Sjögren's syndrome and 43% with APS. Compared to those without 2nd AI, patients with 2nd AI differ in age, sex, race and treatment at last visit. INTEPRETATION: These differences may have important implications for understanding treatments, outcomes and mechanisms of SLE and related diseases.