Why is a prenatal Group B Streptococcus (GBS) test only valid for 5 weeks?

From the Guidelines

A prenatal Group B Streptococcus (GBS) test result is only valid for 5 weeks because the colonization status of a pregnant woman can change over time, as stated in the 2010 guidelines from the CDC 1. After this period, the test results may no longer accurately reflect the current GBS status, potentially leading to inappropriate management during labor. GBS is a bacterium that can temporarily colonize the vaginal and rectal areas, with colonization patterns that fluctuate throughout pregnancy. The standard protocol is to perform GBS screening between 36-37 weeks of gestation, as this timeframe provides the most reliable prediction of colonization status at the time of delivery. If more than 5 weeks pass between testing and delivery, the woman's GBS status may have changed, either from positive to negative or negative to positive. This time limitation ensures that prophylactic antibiotics (typically intravenous penicillin G or ampicillin, or clindamycin for penicillin-allergic patients) are appropriately administered to women who are truly colonized at delivery, reducing the risk of early-onset GBS disease in newborns while avoiding unnecessary antibiotic exposure in women who are not colonized.

Some key points to consider:

• The CDC guidelines from 2010 1 and 2002 1 emphasize the importance of GBS screening and intrapartum antibiotic prophylaxis in preventing early-onset GBS disease.
• The guidelines recommend that a negative GBS screen is considered valid for 5 weeks, and if a patient is re-admitted with signs and symptoms of preterm labor and had a negative GBS screen >5 weeks prior, she should be rescreened and managed according to the algorithm at that time 1.
• The use of intrapartum antibiotic prophylaxis has been shown to reduce the risk of early-onset GBS disease in newborns, and the guidelines provide recommendations for the administration of prophylactic antibiotics based on the results of GBS screening and other risk factors 1.

From the Research

Group B Streptococcus (GBS) Test Validity

• The validity of a prenatal GBS test is limited to 5 weeks due to the potential for colonization status to change over time 2, 3, 4, 5, 6.
• Studies have shown that GBS colonization can occur at any time during pregnancy, and the risk of transmission to the newborn is highest during labor and delivery 3, 6.
• The Centers for Disease Control and Prevention (CDC) recommends universal vaginal-rectal screening for GBS colonization between 35 and 37 weeks of gestation 2, 4, 5.
• The screening-based strategy involves universal screening of all pregnant women at 35 to 37 weeks' gestation for vaginal and rectal GBS colonization, and recommends intrapartum antibiotic prophylaxis for all GBS carriers 3, 5.

Intrapartum Antibiotic Prophylaxis

• Intrapartum antibiotic prophylaxis is the primary recommended approach to prevent perinatal GBS disease 2, 3, 5, 6.
• Penicillin, ampicillin, or cefazolin are recommended for prophylaxis, with clindamycin and vancomycin reserved for cases of significant maternal penicillin allergy 2, 3, 5, 6.
• The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists recommend intrapartum antibiotic prophylaxis based on antenatal screening for GBS colonization 2.

GBS Colonization and Neonatal Outcomes

• GBS colonization is a major cause of sepsis and meningitis in newborns 4, 6.
• The incidence of early-onset GBS disease has decreased by 70% since the implementation of intrapartum antibiotic prophylaxis 5.
• Studies have shown that intrapartum antibiotic prophylaxis can reduce the risk of early-onset neonatal GBS infection by 80-95% 3, 6.