Relative importance of genetic effects in rheumatoid arthritis: historical cohort study of Danish nationwide twin population

OBJECTIVE: To determine the relative importance of environmental and genetic effects in the development of rheumatoid arthritis. DESIGN: Historical cohort study with record linkage between a twin registry and the Danish discharge registry as well as the Danish national registry of deaths used to estimate completeness. SETTING: Two population based nationwide twin birth cohorts. PARTICIPANTS: 37 338 twins were sent a questionnaire about rheumatic diseases. Self reported rheumatoid arthritis was verified by clinical examination and from medical records. MAIN OUTCOME MEASURES: The probandwise concordance rate of rheumatoid arthritis in monozygotic and dizygotic twins. RESULTS: The response rate was 84.7%. Rheumatoid arthritis was verified in 13 monozygotic and 36 dizygotic twins. There were no concordant monozygotic twin pairs and two concordant dizygotic twin pairs. Based on capture-recapture methods the probability of ascertainment was 78.3%. The probandwise concordance rate was 0 (95% confidence interval 0 to 24.7) in monozygotic twins and 8.8 (1.9 to 23.7) in dizygotic twins. CONCLUSION: Genes are of minor importance in the development of rheumatoid arthritis.