Which blood test would you order for a patient suspected of having ankylosing spondylitis?

Ordering the HLA-B27 test is pertinent for a patient suspected of having ankylosing spondylitis because this blood test detects the presence of the HLA-B27 antigen, which is associated with several autoimmune and inflammatory diseases, including ankylosing spondylitis. A significant proportion of individuals with ankylosing spondylitis, approximately 90% of those with the disease, test positive for the HLA-B27 antigen. However, it is essential to understand that the presence of HLA-B27 is not exclusively indicative of ankylosing spondylitis, as it can also be found in individuals without the condition. Therefore, while the test can support the diagnosis, it should be considered alongside clinical findings and other diagnostic criteria.

The other tests listed are not relevant for diagnosing ankylosing spondylitis. The prostate-specific antigen test is used to screen for prostate cancer and has no relation to spondyloarthritis. Liver function tests are important for assessing liver health but do not provide information about inflammatory conditions like ankylosing spondylitis. A vitamin D level may be ordered for different health concerns but is not specific for diagnosing ankylosing spondylitis. Therefore, among the options presented,