Immunodiagnosis of sexually transmitted disease

Methods for detecting microbial antigens in clinical specimens offer an alternative to culture in the diagnosis of some sexually transmitted diseases. Developers of the immunologic methods are faced with a number of problems in evaluating the new tests. Traditionally, these tests are compared to culture as the "gold standard." Unfortunately, culture for Neisseria gonorrhoeae or Chlamydia trachomatis--the two agents most commonly sought--is considerably less sensitive than 100 percent. Immunologic methods may appear to produce false positives when the paired specimens are actually false-negative cultures. Another source of discordant results is sampling variation. These considerations, however, will not account for all false-positive results. Even the best non-culture methods have a low rate of false-positive results. If a new test has a specificity of 97 percent, it, by definition, yields approximately 3 percent false-positive reactions. In low-prevalence settings this false-positive rate will create problems in interpreting the results. For example, in a population with 3 percent prevalence of infection, a positive result in a 97 percent specificity test could only have a predictive value of 50 percent. Most testing for STD agents is performed in low-prevalence settings. None of the currently available immunodiagnostic procedures has a performance profile that suggests it will be satisfactory for diagnostic use in the low-prevalence setting.