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Familial aggregation of rheumatoid arthritis in The Netherlands: a cross-sectional hospital-based survey. European Consortium on Rheumatoid Arthritis families (ECRAF).

OBJECTIVES
To study the familial aggregation of rheumatoid arthritis (RA) in The Netherlands and to analyse the effect of proband characteristics on the concordance rates for RA. Secondary aims were to compare the characteristics of patients in an early RA inception cohort with those of regular patients and to select Dutch families for the genome-wide scan carried out by the European Consortium on RA families (ECRAF).


METHODS
A cross-sectional, hospital-based survey aimed to identify affected sibpair (ASP) families among our whole RA population. Familial RA, or an ASP family, was defined by the presence of at least two siblings fulfilling the 1987 ACR criteria for RA.


RESULTS
The estimated prevalence for familial RA was 9.8% and similar to that found in previous hospital series. The true-positive reporting rate for RA in sibs was 60%. Sibship size in ASP families (mean +/- S.D. = 7.8+/-3.3) was significantly larger than in the Dutch population. Probands with familial RA were more often rheumatoid factor positive and had a longer follow-up. Male gender and history of joint replacements were associated with higher concordance rates for RA. However, regression analysis showed that, correcting for sibship size, the concordance rate for RA was largely not explained by proband characteristics. Compared to regular RA patients, our inception cohort encompassed more male and/or rheumatoid factor-negative patients, but had similar performance in the study and rate of familial aggregation.


CONCLUSIONS
The familial aggregation of RA in The Netherlands is not increased and occurs preferentially in large sibships. Among proband characteristics, sibship size is most clearly related to the recurrence of RA in particular families. Patients' recognition of RA manifestations in relatives is not optimal.