The risk of sacroiliitis in B27 positive persons: a reappraisal.

The prevalence of sacroiliitis was assessed clinically and radiologically in 261/323 1st degree relatives of 47 B27 positive probands who had ankylosing spondylitis (AS). Of the 151 relatives who were B27 positive, 23 had bilateral sacroiliitis (asymptomatic in 4), 2 had had Reiter's syndrome (RS) and 3 acute anterior uveitis (AAU). Similar assessment of 148/182 relatives of 34 B27 positive patients without demonstrable sacroiliitis (including 15 who had had RS and 11 AAU) showed 84 to be B27 positive. Only 1 had sacroiliitis, 2 had RS, 2 AAU and 2 chronic polyarthritis. Features of this study important in determining prevalence are the relatively high completion rate and the concurrent blind reading of radiographs of both B27 positive and negative subjects. AS is found in some B27 positive families, but not in others.