Evaluation of two rapid group B streptococcal antigen tests in labor and delivery patients

Two rapid group B streptococcal antigen tests were compared with nonselective blood agar culture in 1062 unselected patients admitted to labor and delivery. Vaginal specimens taken from each patient on admission were used to perform each of two rapid tests and corresponding cultures. The rapid tests were the Streptex latex agglutination assay and the Equate Strep B test, which uses a solid-phase immunoassay. Overall, 105 patients (9.9%) had at least one positive culture. The sensitivities for the rapid tests were 15.1% for Streptex and 21.5% for Equate. Specificities were 99.3 and 98.7%, respectively. Sensitivity was minimally increased in the setting of ruptured membranes for both tests. Likewise, use of separate swabs for streaking the culture plate and performing the rapid test increased the sensitivity, but this was not significant for either test. In control experiments, the limit of sensitivity of both rapid tests was 5 x 10(6) colony-forming units. We conclude that at present, these tests are not sensitive enough for routine use in this type of clinical setting.