Longitudinal Study of Group B Streptococcus Carriage in Pregnancy

Abstract

Objective This prospective study was designed to 1) determine the prevalence of group B streptococcus (GBS) in our obstetric population and 2) evaluate the predictive value of lower vaginal/perianal GBS cultures obtained in each trimester of pregnancy relative to GBS culture status at delivery. Methods Lower vaginal/perianal GBS cultures were obtained in the first trimester, at 26–28 weeks, at 37 weeks, and on admission for delivery. The investigators were blinded to the results of all cultures except those obtained at 37 weeks. The sensitivity, specificity, and positive (PPV) and negative predictive values (NPV) of each group of cultures with respect to culture status at delivery were determined, and the pattern of GBS carriage in our patients was delineated. Results Nine hundred seventy-three patients participated in this longitudinal study. The prevalence of GBS carriage was 14.0% in the first trimester, 13.9% at 26–28 weeks, 12.4% at 37 weeks, and 12.1% at delivery. GBS carriage was continuous (all 4 cultures positive) in 3.8% and identified on a single culture only in 7.8%. Sensitivity (S1), specificity (S2), PPV, and NPV for each set of antepartum cultures with respect to culture status at delivery were as tabled: Delivery GBS status (%) First trimester x S1=49.5; First trimester x S2=91.8; First trimester x PPV =45.5; First trimester x NPV=92.9 26–28 weeks x S1=68.4; 26–28 weeks x S2=93.9; 26–28 weeks x PPV=60.4; 26–28 weeks x NPV=95.6 37 weeks x S1=63.3; 37 weeks x S2=94.5; 37 weeks x PPV=61.3; 37 weeks x NPV=95.0 Conclusions The pattern of GBS carriage in pregnancy is highly variable. Regardless of when antenatal GBS cultures are done, they serve as poor predictors of maternal GBS carriage at delivery. Infect. Dis. Obstet. Gynecol. 5:237–243, 1997. © 1997 Wiley-Liss, Inc.