Effect of disease severity on rheumatoid arthritis concordance in same sexed siblings.

To assess the factors in a proband with rheumatoid arthritis (RA) that might predict the occurrence of the disease in siblings, 240 same sexed sibships (190 female, 50 male) in which the proband had classical or definite RA were clinically and immunologically documented. Sibship concordance rates were consistently higher for features of severe disease in the proband, reaching statistical significance for a clinical score of disease severity (the SS index). This trend for increasing disease severity to be associated with increasing sibship concordance rates could not be accounted for by age or disease duration of the proband. These results suggest that siblings of probands with severe RA are at greater risk of developing RA than those of probands with mild disease.