Which laboratory tests are most useful in the initial evaluation of a patient suspected to have ankylosing spondylitis?

The most useful laboratory tests in the initial evaluation of a patient suspected to have ankylosing spondylitis are:

• Human leukocyte antigen B27 (HLA-B27) testing: This test is recommended if the patient has clinical features suggestive of axial spondyloarthritis but does not meet full referral criteria, or if exactly three additional diagnostic criteria are present. A positive HLA-B27 increases the likelihood of spondyloarthritis and supports referral to a rheumatologist for further assessment.
• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR): These inflammatory markers may be measured but normal results do not exclude the diagnosis of ankylosing spondylitis.

Other laboratory tests such as infective antibody status are not routinely recommended for diagnosis. Imaging and clinical criteria remain essential for diagnosis alongside these tests.

Therefore, HLA-B27 testing combined with inflammatory markers (CRP and ESR) are the key laboratory investigations in the initial evaluation of suspected ankylosing spondylitis.

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