This test detects the HLA-B27 antigen. HLA-B27 is found in 90% of patients with ankylosing spondylitis and 80% in Reiter's disease. Ankylosing spondylitis affects 1 in 1000 Caucasians. Ankylosing spondylitis is 10 times more common among individuals with HLA-B27 than individuals without this antigen.