First, HLA-B27 is a perfectly normal gene found in 8% of the Caucasian population. Generally speaking, no more than 2% of people born with this gene will eventually get spondylitis.
Secondly, it is important to note that the HLA-B27 test is not a diagnostic test for ankylosing spondylitis. Also, the association between ankylosing spondylitis and HLA-B27 varies in different ethnic and racial groups. It can be a very strong indicator in that over 95% of people in the Caucasion population who have ankylosing spondylitis test HLA-B27 positive. However, only 50% of African American patients with ankylosing spondylitis possess HLA-B27, and it is close to 80% among ankylosing spondylitis patients from Mediterranean countries.
Since there is no single blood test for ankylosing spondylitis, 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. However, these elevated levels are not present in all AS patients and when they are, it can be from other causes such as anemia, infection and cancer. For example, it is estimated that less than 70% of people with ankylosing spondylitis have a raised ESR level.
Finally, there is no association with ankylosing spondylitis and rheumatoid factor (associated with rheumatoid arthritis) and antinuclear antibodies (associated with lupus).