Diagnostic accuracy of a rapid real-time polymerase chain reaction assay for universal intrapartum group B streptococcus screening.

BACKGROUND: Intrapartum antibiotic prophylaxis is currently given to mothers who test positive for group B streptococcus (GBS) by antenatal culture-based screening, with a risk-based approach for cases with an unknown GBS status. A rapid real-time polymerase chain reaction (PCR) assay for the detection of GBS became available recently, making intrapartum screening possible. We aimed to assess its diagnostic accuracy and to compare it with antenatal screening.
METHODS: We conducted a prospective study in a French hospital. All pregnant women giving birth at the maternity ward were considered for inclusion, except those with planned cesarean delivery, with delivery at <35 weeks gestation, and who received antibiotic therapy before admission. We performed GBS culture (the reference standard) and a molecular GBS test (Xpert GBS; Cepheid) on intrapartum specimens. Decisions about intrapartum antibiotic prophylaxis were based on the current GBS screening by culture at 35-37 weeks gestation.
RESULTS: We prospectively enrolled 968 pregnant women from April 2007 through March 2008. The overall molecular GBS test yield was 89.2%. Among the 863 women with available results, the molecular GBS test had a sensitivity of 98.5%, specificity of 99.6%, positive predictive value of 97.8%, and negative predictive value of 99.7%. The positive predictive value of antenatal culture for identifying colonization status at delivery was low (58.3%), whereas the negative predictive value was imperfect (92.1%).
CONCLUSIONS: This real-time PCR assay is a highly accurate test to identify intrapartum GBS carriers at point of care. This new tool could enhance the exact identification of candidates for intrapartum antibiotic prophylaxis, including women with preterm rupture of membranes or preterm labor.