In colonized women, several risk factors have been identified that increase the probability of early-onset neonatal GBS infection. These factors include: preterm labor, especially when complicated by premature rupture of membranes; intrapartum maternal fever (usually due to chorioamnionitis); rupture of membranes greater than 18 hours before delivery; previous delivery of an infected infant; young age; and black or Hispanic ethnicity. Approximately 25% of colonized women will have one of these risk factors.5-7
These risk factors have a profound impact on neonatal attack rates and mortality. Without the interventions outlined below, the neonatal infection rate is 40% to 50% in the presence of a risk factor and less than 5% in the absence of a risk factor. In infected infants, neonatal mortality approaches 30% to 35% when a maternal risk factor is present, but is less than 5% when risk factors are absent.5-7
3. “What will you do to determine if I am colonized with this organism?”
The current guidelines set forth in the ACOG Committee Opinion recommend that selected high-risk patients (patients with preterm labor or preterm premature rupture of membranes) be tested for GBS at the time of initial presentation. All other women should be tested for GBS during the interval 36 0/7 to 37 6/7 weeks’ gestation.5 Testing at this point in pregnancy is almost 90% sensitive for identifying patients who will be colonized at the time of admission for labor if no more than 5 weeks elapse between the time the culture is obtained and labor begins. The positive predictive value of this test is 87%, and the negative predictive value is 96%.8
ACOG’s previous guidelines provided for testing at 35 rather than 36 weeks. The change in the recommendations was based on 2 factors. First, all women with unknown GBS status who may deliver before 37 weeks already should be targeted for prophylaxis. Second, the new 5-week window now will include women who deliver up to 41 weeks’ gestation. Given current obstetric practice in the US, delivery beyond 41 weeks is unlikely.5
At the present time, the best test for identification of GBS colonization is bacteriologic culture. A cotton swab is placed into the lower third of the vagina, streaked along the perineum, and then placed into the rectum. The swab is withdrawn, placed in a culturette tube, and transported to the laboratory. In the laboratory, the swab is cultured for approximately 24 hours in a nutrient broth and then subcultured on a selective blood agar plate. Failure to sample both the vagina and rectum or failure to use selective broth and selective blood agar will reduce the yield of positive cultures by approximately 50%.5-7
In recent years, researchers have become interested in the use of rapid nucleic acid amplification tests for the identification of GBS. These tests perform well if the test protocol provides for an 18- to 24-hour incubation in nutrient broth prior to application of the nucleic acid probe. When the tests are performed without this enrichment phase, sensitivities are inferior to those associated with bacteriologic culture. In addition, because the rapid tests do not isolate the organisms, they do not allow for antibiotic sensitivity testing.5-7
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