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.