The prevalence of ankylosing spondylitis among B27 positive normal individuals--a reassessment.

Previous workers reported symptomatic, but undiagnosed, ankylosing spondylitis (AS) in some 20% of blood donors with HLA B27. As part of an ongoing population study, we compared 139 B27 positive individuals with 128 controls and found no differences in spinal mobility or back pain. There were no differences when all available radiographs were compared. No case of AS was identified. It is concluded that AS occurs in far less than 20% of B27 positive individuals and that the prevalence of AS is of the order predicted by conventional epidemiological surveys. Although HLA typing can be helpful in excluding AS suspected on clinical grounds, it cannot be used to confirm the diagnosis.