Rapid intrapartum test (Strep B OIA) and prenatal cultures for identification of group B streptococcal carriers at delivery: a prospective study

Background: The efficacy of the prenatal screening-based approach recommended by the CDC to prevent neonatal GBS diseases could be improved by using a good rapid test performed at the onset of labor. To assess the Strep B OIA® test (Biostar, Boulder, Co), completed in 30 minutes, we initiated a 6-center study to compare it with prenatal screening cultures to identify GBS carriers at delivery or opportunities to initiate intrapartum antibioprophylaxis (IAP). Methods: For a total of 539 pregnant women included in the study, pairs of vaginal/anal specimens collected at 35-37 weeks and intrapartum vaginal specimens were plated onto colistin-nalidixic agar and then inoculated into selective LIM broth for the detection of GBS. Furthermore, on each intrapartum vaginal swab a Strep B OIA test was performed. Results: GBS were recovered in culture from 89 prenatal screenings (17%) and from 71 specimens collected at delivery (13%). Strep B OIA test identified 48 positive specimens (9%). Respectively, for the identification of GBS carriers at delivery, sensitivity, specificity, positive and negative predictive values for prenatal screening cultures were 69%, 91%, 55% and 95% and for Strep B OIA tests they were 63%, 99%, 94% and 95%. Evaluating opportunities to start an IAP, based on Strep OIA test versus prenatal screening cultures, 45 IAP vs. 49 would have been useful, 3 vs. 40 useless (P< 0,001) and 26 vs. 22 missed. Conclusion: To identify GBS carriers, the Strep B OIA test, performed at the time of onset of labor is equally sensitive to prenatal screening cultures and would allow a highly significant reduction of useless IAP.