[Usefulness of HLA-B27 in the diagnosis of ankylosing spondylitis: positive and negative predictive value and the probability ratio of diseases].

The authors discuss the usefulness of the HLA-B27 test in the diagnosis of ankylosing spondylitis (AS). Sensitivity, Specificity, Predictive Value of a Negative Test, Predictive Value of a Positive Test, and Likelihood Ratio are defined and illustrated through the example involving HLA-B27 and AS. The HLA-B27 cannot be used as a screening test for the diagnosis of AS when evaluating an asymptomatic population, however the test is most useful in assisting the diagnosis of AS when used by physicians who understand the principles of probability.