Second, it is important to note that the HLA-B27 test is not a diagnostic test for ankylosing spondylitis. Also, the association between AS and HLA-B27 varies among different ethnic and racial groups. It can be a very strong indicator in that more than 95 percent of people in the Caucasian population who have AS test positive for HLA-B27. However, close to 80 percent of AS patients from Mediterranean countries and only 50 percent of African American patients with AS are HLA-B27 positive.
Since there is no single blood test for AS, laboratory work may, or may not, be of help. Elevated erythrocyte sedimentation rate (ESR), also known as SED rate, and C-reactive protein (CRP) are common indicators of inflammation. Elevated levels of these markers, however, are not present in all AS patients and, when they are, it can be from other causes such as anemia, infection, or cancer. For example, it is estimated that less than 70 percent of people with AS have a raised ESR level.
Finally, there is no association between AS and rheumatoid factor (associated with rheumatoid arthritis) and antinuclear antibodies (associated with lupus.)