Rapid Intrapartum Screening for Group B Streptococci: How Efficient Is It Really?

A new study confirmed the value of real-time polymerase chain reaction (PCR) assay as a rapid method of screening for group B streptococci (GBS) colonization during parturition.¹ Using real-time automated PCR assay, DNA amplification testing, and standard culture, Edwards and colleagues¹ comparatively looked at the detection of GBS colonization in women who were in the 35th to 37th week of pregnancy and in women who were about to give birth. A true-positive result was defined as a positive molecular test and a positive culture finding. Compared with culture, the sensitivity rate of PCR was 91.1%, the specificity was 96.0%, the predictive value was 87.8%, the negative predictive value was 97.1%, and the accuracy was 94.8%. As anticipated, PCR assay was more sensitive than DNA amplification testing (91.1% vs 79.3%). Neither specificity, positive predictive value, nor detection of GBS prevalence was statistically divergent.

The study authors commented that the "performance characteristics of the PCR assay exceed the threshold recommended by the Centers for Disease Control and Prevention when compared with culture." They stated that the test is "robust" enough to use during parturition. Caveats, however, still need to be considered in intrapartum GBS screening using PCR assay.

A significant problem is that a number of substances- specifically blood, feces, meconium, and amniotic fluid- can interfere with assay results. If real-time PCR technology were routinely used in the intrapartum period, a significant number of critical test specimens would be invalid. This observation, however, was not made in the study by Edwards and colleagues because of the manner in which specimen collection and selection was made. A second problem is that real-time PCR technology applied in the intrapartum period may not be cost-effective if only 1 or 2 specimens are to be assayed. PCR assay requires experienced laboratory technicians and a specialized facility because of the assay's marked sensitivity and the potential for environmental contamination. PCR assay performed in bulk, however, can be cost-effective. In the study by Edwards and colleagues, analysis of comparative costs was neglected.

CDC guidelines recommend that all pregnant women be screened for colonization of GBS at 35 to 37 weeks in an effort to predict whether substantial colonization will be present at term. Because of the concern that GBS will elude detection or that acquisition of GBS will occur in the interval between testing and parturition, new emphasis is being given to detecting and treating GBS colonization within the intrapartum period. Medicolegally, there is a zero tolerance for the occurrence of group B streptococcal infection in a newborn; hence, the renewed advocacy for intrapartum monitoring.

The predictive value of antepartum cultures performed within 6 weeks of delivery to provide insight into GBS colonization at labor is between 67% and 87%.^2,3 Antepartum identification of GBS colonization can be adversely influenced by the diagnostic test used, media selection, and the number of organisms present.⁴ One or more of these factors can produce a false-negative test result in a pregnant woman who harbors GBS in her vaginal bacterial flora.

Problems with culture-based diagnostic testing include inconsistent availability of appropriate culture media, inappropriate specimen collection technique, and the time required to obtain test results. Culture results are usually available in 48 to 60 hours, whereas results of PCR assay can be available in less than an hour and a half. The method recommended by the CDC is to combine vaginal and rectal specimens for culture. As a result, rectal colonization cannot be distinguished from vaginal colonization. This outcome may lead to inefficiency in evaluating potential perinatal risk of group B streptococcal infection because GBS are not uncommon constituents of fecal bacterial flora; the greater concern is whether GBS are constituents of the vaginal bacterial flora in a pregnant woman.

The quest for increased test sensitivity as well as specificity has been progressive. Before PCR assay and DNA amplification testing, use of Gram stain smears gave way to culture testing for GBS. The adaptation of PCR technology to real-time testing now makes it possible for a large number of specimens to be analyzed in a single run. The best projected use of real-time PCR technology may be mass screening of specimens collected between 35 and 37 weeks of gestation. The value of a separately collected rectal swab specimen tested with a PCR test will be of concern, given that PCR inhibitors are embedded in feces.

References:

• Edwards RK, Novak-Weekley SM, Koty PP, et al. Rapid group B streptococci screening using a real-time polymerase chain reaction assay. Obstet Gynecol. 2008;111:1335-1341.

• Yancey MK, Schuchat A, Brown LK, et al. The accuracy of late antenatal screening cultures in predicting genital group B streptococcal colonization at delivery. Obstet Gynecol. 1996;88:811-815.

• Edwards RK, Clark P, Duff P. Intrapartum antibiotic prophylaxis 2: positive predictive value of antenatal group B streptococci cultures and antibiotic susceptibility of clinical isolates. Obstet Gynecol. 2002;100:540-544.

• Boyer KM, Gadzala CA, Kelly PD, et al. Selective intrapartum chemoprophylaxis of neonatal group B streptococcal early-onset disease. II. Predictive value of prenatal cultures. J Infect Dis. 1983;148:802-809.