Clinical Inquiries

How best to manage the patient in term labor whose group B strep status is unknown?

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EVIDENCE-BASED ANSWER

Monitor the patient and treat her with intrapartum chemoprophylaxis based on identified risk factors, unless a rapid, highly sensitive (≥85%) polymerase chain reaction (PCR) test is immediately available to evaluate for group B Streptococcus (GBS) (strength of recommendation: B, inconsistent or limited quality evidence).

Evidence summary

GBS infection is a leading cause of neonatal bacteremia, pneumonia, and meningitis.1 Approximately 10% to 30% of pregnant women are colonized with GBS in the vagina or rectum.2,3 Giving intrapartum antibiotics to women at high risk for GBS colonization (TABLE 1) has significantly reduced the incidence of neonatal sepsis, from approximately 2 cases to 0.5 cases per 1000 live births.1,3

TABLE 1
Risk factors for GBS

Temperature >100.4°F
Rupture of membranes >18 hours
Preterm labor <37 weeks
GBS bacteriuria anytime during pregnancy
Previous infant with early-onset GBS infection
GBS, group B Streptococcus.
Source: 2002 CDC guidelines5 in consensus with the American Academy of Pediatrics and the American College of Obstetrics and Gynecology.

Universal screening is the way to go

A multistate retrospective cohort study (N=5144) concluded that universal screening by culture for GBS at 35 to 37 weeks’ gestation is more effective than treating patients based on risk-factor criteria (relative risk [RR]=0.46; 95% confidence interval [CI], 0.36-0.60).4 The Centers for Disease Control and Prevention’s (CDC) 2002 revised guidelines recommend universal screening by culture at 35 to 37 weeks, with 2 exceptions: women who had GBS bacteriuria during the current pregnancy and women who have given birth to an infant with invasive GBS disease.5 The GBS status of 5% to 10% of term patients remains unknown at onset of labor because of inadequate prenatal care or out-of-date GBS culture results (>5 weeks).6,7

PCR is the most accurate rapid test for GBS

A 2006 systematic review of 29 prospective studies found the PCR test to be the most accurate rapid assessment for GBS status at onset of labor.8 The test was 96% sensitive (95% CI, 0.88-0.99) and 98% specific (95% CI, 0.96-0.99) compared with optical immunoassay, DNA hybridization, enzyme immunoassay, latex agglutination, and Islam starch medium tests (TABLE 2).8 Culture, usually on selective media, was the gold standard.

The rapid PCR test takes about 40 minutes, making it useful to determine the need for antibiotic prophylaxis in laboring, full-term, GBS-status-unknown women. If a rapid PCR test isn’t available, a woman with unknown GBS status and 1 or more of the CDC risk factor criteria should receive intrapartum antibiotics.5

TABLE 2
How accurate are intrapartum tests for GBS?

TESTLR+LR–TYPICAL DURATION (MIN)
Rapid PCR38.80.0640-100
Optical immunoassay16.010.3530
DNA hybridizationNANA60-1440
Enzyme immunoassay9.370.785-10
Latex agglutination83.180.4370-85
Islam starch medium28.330.57120-1400
GBS, group B Streptococcus; LR+, positive likelihood ratio; LR–, negative likelihood ratio; PCR, polymerase chain reaction.
Tests with an LR >10 effectively rule in disease; tests with an LR <0.1 effectively rule out disease.
Source: Honest H et al.8

Evidence-based answers from the Family Physicians Inquiries Network

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