HLA-B27 and the seronegative spondyloarthropathies. Prevalence of B27 positivity varies by disease and subtype. Ankylosing spondylitis carries the highest association (90%). In Reactive Arthritis, B27 predicts a more severe and prolonged course. Peripheral PsA is often B27-negative; axial involvement is B27-associated. Acute anterior uveitis — the most common extra-articular feature across all SpA — is B27-positive in ~50% of cases.