What is a GBS Carrier?
A GBS carrier is a pregnant woman who has Group B Streptococcus (Streptococcus agalactiae) colonizing her vaginal and/or rectal areas asymptomatically, identified through prenatal screening cultures, and who poses a risk for vertical transmission to her newborn during labor and delivery. 1
Definition and Colonization Pattern
GBS colonizes the lower genital tract of approximately 10-30% of pregnant women as an asymptomatic member of the gastrointestinal and vaginal flora. 2, 3
The gastrointestinal tract serves as the primary reservoir for GBS, with secondary spread to the genitourinary tract. 4, 2
Colonization is asymptomatic — carriers have no symptoms or signs of infection but harbor the bacteria in their rectovaginal area. 1, 3
Clinical Significance of Carrier Status
Newborns born to GBS carriers are at increased risk for early-onset GBS disease, which manifests as bacteremia, pneumonia, or meningitis within the first 7 days of life. 1
Women with GBS colonization are more than 25 times more likely to deliver infants with early-onset GBS disease compared to those with negative cultures. 2
Among infants who developed early-onset GBS disease, 88% were born to mothers who were detected prenatally as carriers. 1
Identification of Carriers
Optimal screening technique requires:
Timing: Cultures collected at 35-37 weeks' gestation provide the best correlation with intrapartum colonization status. 4, 1
Sites: Both vaginal introitus and rectum must be swabbed — vaginal and rectal specimens are more sensitive than cervical specimens alone. 1
Medium: Selective broth media (such as SBM broth or Lim broth) should be used for optimal detection. 1
When cultures are obtained ≤5 weeks before delivery, there is 100% concordance with intrapartum culture status (no false-negative or false-positive results). 1
Predictive Value and Duration of Carriage
A single positive GBS culture during pregnancy is 67% predictive of a positive culture at delivery, with estimated sensitivity of 70.0% and specificity of 90.4%. 1
The duration of GBS carriage is unpredictable — women can gain or lose colonization status over time, which is why screening timing is critical. 1
Only 7.4% of women with a negative culture at 26-28 weeks were found to carry GBS at delivery, emphasizing the importance of late-pregnancy screening. 1
Risk Stratification for Carriers
High-risk GBS carriers who require intrapartum antibiotic prophylaxis include those with:
- Rupture of membranes >12 hours before delivery 1
- Onset of labor or membrane rupture at <37 weeks' gestation 1
- Intrapartum fever (temperature >99.5°F or >37.5°C) 1
- Previous delivery of an infant with GBS disease 1
- Multiple-gestation pregnancy 1
- GBS bacteriuria during current pregnancy (indicates heavy colonization) 2
Common Pitfalls
Do not collect screening cultures from the cervix alone — this significantly reduces sensitivity compared to rectovaginal sampling. 1
Do not perform speculum examination for screening cultures — external swabs of the vaginal introitus and rectum are sufficient and more sensitive. 1
Carrier status does not mean active infection — these women are asymptomatic colonizers, not infected patients requiring treatment outside of labor. 1, 3
Screening too early in pregnancy (before 35 weeks) reduces predictive accuracy for colonization status at delivery. 1