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Prenatal Group B Strep Screening Test

Last updated May 4, 2018

A Prenatal Group B Strep Screen Test is regularly performed on an expectant mother, during late pregnancy. The goal is to determine, if Strep B is present and (if present) whether it has the ability to infect a newborn child.


What are the other Names for this Test? (Equivalent Terms)

  • Beta-Hemolytic Strep Culture Test
  • GBS Test
  • Strep B Screening Test

What is Prenatal Group B Strep Screening Test? (Background Information)

  • Streptococcus belongs to a bacterial genus. Group B Streptococcus (or Strep B) is a common name for Streptococcus agalactiae, a subset of Streptococci. It is so named, because it displays the group B antigen, similar to how red blood cells display A or B antigens
  • Strep B is often present in the intestines and vagina of healthy individuals. It is not a cause for alarm, because a healthy immune system and competing microbes prevent Streptococcus from overgrowing
  • Newborn babies, however, lack a developed immune system and competing microflora (a complex set of microorganisms in the gut). Thus, they are susceptible to streptococcal infections
  • An infection is most likely to occur during passage of the baby, through the birth canal, during delivery. This is the period when the newborns are exposed to the mother’s natural microbes
  • A Prenatal Group B Strep Screen Test is regularly performed on an expectant mother, during late pregnancy. The goal is to determine, if Strep B is present and (if present) whether it has the ability to infect a newborn child

What are the Clinical Indications for performing the Prenatal Group B Strep Screening Test?

The following are clinical indicators for performing a Prenatal Group B Strep Screening Test:

  • Expectant mothers, 35-37 weeks into pregnancy
  • Fever during labor (intrapartum fever)
  • Previous delivery of a newborn with a group B streptococcal infection
  • The presence of Strep B in the urine (streptococcal bacteriuria), during pregnancy
  • “Water breaking” or rupturing of membranes/amniotic sac (termed as amniorrhexis), 18 hours or earlier before delivery

How is the Specimen Collected for Prenatal Group B Strep Screening Test?

Sample required: Vaginal or rectal culture

Process: Swabbing of vagina (vaginal culture) or rectum (rectal culture), using a sterile instrument.

Preparation required: None

What is the Significance of the Prenatal Group B Strep Screening Test Result?

A lack of Streptococcus B in the test report requires no further action steps. However, the presence of Strep B may indicate the following in the newborn:

  • Pneumonia
  • Meningitis
  • Sepsis
  • The need to administer intravenous antibiotics (intrapartum prophylaxis)

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

References and Information Sources used for the Article:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: March 12, 2014
Last updated: May 4, 2018