Incidence of rheumatoid arthritis in a 10-year follow-up study of extended pedigree multicase families.

A 10-year follow-up study was undertaken of 17 multicase RA families with extended pedigrees to (i) determine the incidence of RA in the previously unaffected members and (ii) to assess the stability of diagnosis as defined by the 1958 and 1987 ARA criteria. In all, eight individuals developed RA. Of these, six were surviving at 10 years--four first degree (FDR), one second degree (SDR) and one non-blood relative (NBR) of the proband equivalent to incidence densities of 9, 3, and 3 per 1000 person years of observation respectively. These are substantially greater than estimates for both the general population and a DR1/4 positive population. The risk of RA is greatest in FDR and this is likely to be due to both shared inherited and environmental factors. The risk in NBR, who share no genetic material is of similar magnitude to more distant blood relatives. Definite/classical RA is a stable diagnosis over time in the majority of cases unlike probable RA which may herald definite RA but usually does not.