Optimization of a rapid diagnostic test for detection of group B streptococcus from antepartum patients

We analyzed the performance of a new rapid diagnostic test for use in determining group B streptococcus colonization in pregnancy. Vaginal-rectal specimens were compared by the rapid test, a commercial laboratory culture result, and an in-house culture. Of 150 patient samples, 72 were positive by the rapid test, giving a prevalence of 48.0% versus 24.7% by traditional culture. Characterization of these results showed cross-reactivity with Enterococcus. The addition of bacitracin reduced this interference, and when reanalyzed, a colonization rate of 31.3% was found (P = 0.3961, chi-square), as well as a sensitivity of 100% (95% confidence interval [CI] 89.1-100) and a specificity of 93.6% (95% CI 86.9-97.2). The addition of bacitracin greatly improves the reliability of this diagnostic test and demonstrates a novel approach to reduce interference. An accurate determination of the test's sensitivity and specificity, however, awaits enrollment of the remaining subjects.