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Coexistence of rheumatoid arthritis and ankylosing spondylitis--report of 10 cases.

The distinction between rheumatoid arthritis (RA) and ankylosing spondylitis (AS) has hitherto relied on supporting evidence of characteristic radiological changes in the sacroiliac joints, together with the Rose-Waaler and Latex tests for rheumatoid factor (RF). This distinction has remained incomplete since some 30 per cent of patients with RA may have sacroiliitis, a similar proportion having negative routine tests for RF. The identification of the HLA B27 antigen, present in 90 per cent of cases of AS and six per cent of the normal population, has enabled a number of cases to be recognized in which both diseases appear to co-exist. Ten cases are described in which either RA appears to have developed in patients with AS, or AS in patients with RA. They all fulfil the ARA diagnostic criteria for classical RA, and the criteria for classical AS. The likelihood of these two diseases occurring by chance in an individual might be of the order of 1:50,000 to 1:200,000.