Intrapartum Tests for Group B Streptococcus

Abstract & Commentary

By John C. Hobbins, MD, Professor, Department of Obstetrics and Gynecology, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.

Professor Hobbins reports no financial relationship to this field of study.

Synopsis: Two rapid screening tests for group B streptococcus are reasonably sensitive, but the most predictive of the two, a polymerase chain reaction, may still not have a short enough turnaround time to be of value in the contemporary management of intrapartum patients, which, according to updated CDC guidelines described below, requires at least 4 hours of antibiotics before delivery.

Source: Daniels JP, et al. Intrapartum tests for group B streptococcus: Accuracy and acceptability of screening. BJOG 2010 Oct 13; Epub ahead of print; doi:10.1111/j.1471-0528.2010.02725.

The rate of sepsis with group b streptococcus (gbs) in neonates of colonized mothers is now less than 1%. However, the devastating effects in affected babies are well worth preventing with a rational regimen of prophylaxis, and the strategy published by the Centers for Disease Control and Prevention (CDC) in 20021 seems to have decreased neonatal GBS sepsis by 50%-80%.2 This protocol calls for screening everyone at 35-37 weeks and treating those with positive cultures with intrapartum antibiotic prophylaxis (IAP), along with those with some other risk factors described below.

Recently, rapid assays have emerged that have the potential to guide the practitioner in choosing who to treat with IAP, but some of the kinks need to be worked out. A paper published in the British Journal of Obstetrics and Gynecology may help put into proper perspective the benefits of two rapid tests for GBS in intrapartum patients. The two assays that were evaluated were a polymerase chain reaction (PCR) and an optical immunoassay (OIA). The authors were interested in their accuracy in predicting GBS, as well as their clinical efficacy based on how quickly the answers were available.

Over a 2-year period the authors reviewed intrapartum data on patients who were enrolled in the study early in pregnancy. Once these patients were in labor or being induced they had vaginal and rectal samples taken with two sets of three swabs fastened together during sampling, but separated later so that each of the three swabs would be tested for GBS via PCR, OIA, or, the ultimate answer, enriched culture methodology. Neonates were tested through swabbing of their ear canals. Clinicians were blind to the results and generally used a uniform protocol for IAP, based on risk factors for GBS.

The results were illuminating. In the end, the authors had useful data on 1394 patients, 22% of whom had one or more risk factors. Positive GBS cultures were found in 21% of all mothers, 29% in those with risk factors, and 19% in those without. Neonatal colonization occurred overall in 8.5%, in 36% of colonized women, and in 1% of culture-negative women. Three babies had active infection and all survived. Despite the recommended protocol, only 71% of those with a definite indication for IAP and 50% with any risk factor for GBS received antibiotics.

OIA was the quicker method, with a mean sampling-to-answer time of 38.8 minutes vs 80.8 minutes for PCR. However, the accuracy for predicting a positive GBS culture was more sensitive with the PCR (84% vs 72%) and more specific (87% vs 57%). Rectal PCR performed better than vaginal PCR, with a sensitivity of 71% vs 58%. A positive vaginal or rectal PCR gave a 65% probability for a positive GBS culture, and a negative test had a probability of 5%. There was a higher maternal GBS prevalence for those with risk factors (29% vs 19%), but those with risk factors had heavier growth in the culture. After adjusting for the effect of antibiotics, the odds of neonatal colonization were far lower (odds ratio = 0.22) when IAP was given > 4 hours, compared with a nonsignificant effect when antibiotics were given < 4 hours prior to delivery.


The authors found that, although PCR was a more accurate predictor of GBS colonization, a lag of 80 minutes may be too long in some cases, since there was a negligible effect of IAP when given less than 4 hours before delivery. The other findings provide useful information when used in combination with the conclusions of other studies.

So, where do we stand now on GBS screening? In Great Britain, uniform screening at 35-37 weeks is not the standard, and the clinicians in the study used risk factors alone for consideration of IAP. These were:

  1. Patients with a previous child with GBS infection
  2. Those with suspected chorioamnionitis
  3. Those with preterm premature rupture of the membranes
  4. Those with GBS bacteruria
  5. Those with prolonged rupture of membranes in term pregnancies (> 18 hours)
  6. Those mothers with a temperature > 100.4° F.

The CDC has come out this month with revamped recommendations that again involve screening all pregnant women at 35-37 weeks with vaginal/rectal cultures.3 An exception would be a woman who had a positive urine culture (and who would automatically get IAP). Although screen-positive patients having C-sections with intact membranes would not need IAP, virtually all other screen-positive patients would. The new guidelines indicate that patients with a previous history of neonatal sepsis would automatically be offered IAP, and would not need to be screened. The new guidelines also suggest IAP for patients with the other risk factors listed above. Patients admitted with preterm labor should be cultured and treated, but the antibiotics could be stopped if the cultures return as negative.

The antibiotic regimen would include penicillin G, 5 million units IV followed by 2.5 million units IV q 4h. Another option would be ampicillin 2 g IV, followed by 1 g IV q 4h. In patients with penicillin allergy, cefazolin (if there was a low risk for anaphylaxis) and clindamycin or erythromycin (if there is a high risk for anaphylaxis) could be substituted. If antibiotic sensitivities are available, vancomycin can be used if the bacteria are resistant to clindamycin or erythromycin.

One would hope that rapid, cheap tests for GBS will become available soon that will allow better selection of truly vulnerable patients/neonates. This would allow us to avoid the creation of super-resistant strains of bacteria from shotgun administration of antibiotics to those who really do not need them.


  1. Schrag S, et al. Prevention of perinatal group B strep-tococcal disease. MMWR Recomm Rep 2002;51(RR-11):1-22.
  2. Van Dyke MK, et al. Evaluation of universal antenatal screening for group B streptococcus. N Engl J Med 2009;360:2626-2636.
  3. Prevention of Perinatal Group B Streptococcal Disease: Revised Guidelines for CDC, 2010. MMWR Recomm Rep 2010;59(RR-10):1-36.