Intrapartum Rapid GBS Testing in Patients Presenting With Threatened Preterm Labor

Group B streptococcus (GBS) or Streptococcus agalactiae is a gram-positive bacterium that colonizes 10-40% of maternal gastrointestinal and urogenital tract sites. Maternal colonization remains the primary risk factor and the leading cause of early onset GBS disease in infants in the United States. Transmission of GBS to the neonate in early onset GBS cases occurs at the time of labor and delivery, with a transmission rate of 52.5% if no intrapartum antibiotics are used. Of those neonates, 1-2% term infants and 8% of preterm infants will develop early onset disease.

The Centers for Disease Control (CDC) recommends universal screening at 35-37 weeks via culture of the vagina and rectum. If this is performed ≤5 weeks before delivery, it has a sensitivity of 85% and a negative predictive value of 95-98%. There is a downside to screening remote from delivery however; vaginal GBS colonization fluctuates in the same woman over time, thus rendering possibly inaccurate GBS results. It has been reported that at least 10% of antenatal GBS negative women turned positive at the time of labor. This may suggest that screening at the time of delivery is a more accurate method of predicting actual GBS colonization status. In fact, a majority of neonatal GBS sepsis occurs in infants born to mothers with a negative antepartum screening culture.

Currently, a standard GBS culture may take up to 3 days to obtain results. A rapid diagnostic test has more recently been studied as a possible method of GBS screening - real-time polymerase chain reaction (RT-PCR). Prior studies of RT-PCR, specifically the Cepheid GeneXpert GBS assay used at Miller's Children's and Women's Hospital, have reported sensitivity from 85-98.5% and specificity of 96-99.6% using data from term gestations. The CDC currently permits the use of RT-PCR as a rapid screening test for those with unknown status at term.

Several reports demonstrate that RT-PCR is a rapid, more sensitive method than standard culture for determining the intrapartum GBS colonization status. Some studies have also demonstrated the ability of RT-PCR to identify patients who would otherwise be missed by traditional GBS culture. A study by Mueller et al demonstrated that out of 64 patients with positive RT-PCR results, 10 were actually negative on culture. A cost-effectiveness analysis has demonstrated that PCR intrapartum screening strategy is not any less cost-effective than traditional culture and confers a significant decrease in early onset GBS disease in term gestations.

Preterm infants suffer the highest rate of mortality from GBS infection, with up to 30% mortality in those < 33 weeks affected by GBS sepsis. Identifying GBS colonization is thus imperative in the 7-11% of all pregnancies affected by preterm labor, given that they will not have undergone universal screening yet (which typically occurs at 35-37 weeks). While the CDC recommends giving antibiotics to patients with unknown GBS status at substantial risk for preterm delivery, implementation of this recommendation is poor.