Should I be tested for Group B Streptococcus (GBS)?
The 2010 CDC Guidelines recommend all pregnant women should be tested for Group
B Strep bacteria late in pregnancy, between 35 and 37 weeks' gestation.
All women who test positive for the bacteria should be given antibiotics during
labor to help protect newborn baby from infection.
To find out more on how to protect your baby from Group B Streptococcus infection
and early onset disease, visit www.cdc.gov
Frequently Asked Questions
Q. When is the best time to screen for GBS in pregnancy?
A. Studies have shown that testing within 5 weeks of delivery is most accurate at
predicting GBS colonization status at delivery. The 2010 guidelines published by
CDC state that screening should be done between 35-37 weeks' gestation.1
Q. To screen for GBS, do I have to collect a vaginal swab, a rectal swab, or both?
A. Current guidelines require a vaginal-rectal swab because the sensitivity is highest
for this specimen type.1
Q. The broth enrichment step delays test results by 24 hours. What advantage does
it offer?
A. Broth enrichment greatly enhances the recovery of GBS thus improving the sensitivity.
The 2010 CDC guidelines state that all samples must be enriched in broth media for
18-24 hours. For GBS disease, an accurate screening result is far more important
than a rapid screening result.1
Q. Culture is the preferred method. What are the drawbacks?
A. The majority of infants that develop GBS disease are born to mothers who tested
negative for Group B Streptococcus. This is due in large part to false-negative
culture results.1,2 The sensitivity of culture is unreliable and is documented
to be as low as 42%.3
The lack of sensitivity can be attributed to several factors including:
- Subjectivity.
- 4% of GBS isolates are nonhemolytic and therefore are undetectable on the culture
plate.1
- Overgrowth of normal flora can inhibit the ability to identify GBS colonies.
Q. What are the advantages of molecular testing for Group B Streptococcus?
A. Recommended by CDC guidelines.
A. Sensitivity of 98.6% means fewer false negatives.
A. Eliminates all subjectivity related to traditional culture methods.
A. Definitive, accurate answers mean appropriate therapy can be administered at
delivery, if needed.
Q. Can a molecular test be done directly from a prenatal swab?
A. No. Based on the recommendations of the CDC, all vaginal-rectal swabs must be
inoculated in selective enrichment broth first and incubated for 18-24 hours prior
to performing any molecular test.
Q. Is there any "rapid screening" for GBS that is acceptable for women who present
in labor with unknown colonization status and no other indications for intrapartum
prophylaxis?
A. There are rapid intrapartum nucleic acid amplification tests such as PCR available
for women with unknown GBS colonization and no other indications for intrapartum
prophylaxis. Women who test negative should be given intrapartum prophylaxis if
they develop one of the risk symptoms. This is because some studies have shown that
the sensitivity of PCR performed directly on vaginal/rectal swabs without an enrichment
step was less than that of culture. Thus, clinicians should remember that in this
circumstance intrapartum risk factors trump negative test results.4
1. Department of Health and Human Services, Centers for Disease Control and Prevention.
Prevention of Perinatal Group B Streptococcal Disease, Revised Guidelines
from CDC, 2010. MMWR 2010;59.
2. M. Van Dayke, Ph.D., et al. 2009. Evaluation of Universal Antenatal Screening
for Group B Streptococcus. The New England Journal of Medicine. 360: pp.
2626-2636.
3. F. Rallu, P. Barriga, C. Scrivo, V. Martel-Laferriere, and C. Laferriere. 2006.
Sensitivities of Antigen Detection and PCR assays Greatly Increased Compared to
that of the Standard Culture Method for Screening for Group B Streptococcus
Carriage in Pregnant Women. Journal of Clinical Microbiology. 44: pp. 725-728.
4. CDC website,
www.cdc.gov/groupbstrep/clinicians/QAs-obstetric.html
