Clinical Scenarios for Group B Streptococcus

In this Clinical Scenario I will break down Group B Streptococcus and illustrate how you can correctly answer the following questions for the United States Medical Licensing Exam Step 2 CK.


Group B streptococcus (GBS) is a bacterial infection that is usually asymptomatic and is identified in 25% of pregnant women. Outside of pregnancy it is of little clinical significance and requires no treatment. However, in pregnancy fetal transmission can occur either during labor or after maternal rupture of membranes and can have serious fetal consequences. Undetected or untreated during labor it may cause neonatal sepsis, pneumonia or meningitis. Thus, a positive result warrants intra-partum treatment to prevent neonatal complications. The drug of choice is Penicillin (PCN) given throughout labor. When a PCN allergy is encountered the drug of choice depends on the type of allergy and culture sensitivities. For the USMLE it is important to be able to answer the following questions.

When is Group B Screening Performed? 

  • Group B streptococcus screening is usually performed on all pregnant patients (few exceptions) by obtaining vaginal-rectal swab between 35-37 weeks of gestation.

Which Pregnant Patients do not Need GBS Screening?

  • The presence of the following requires intra-partum prophylaxis irrespective of the GBS screening result thus, there is no need to perform and these conditions are:
  1.  A positive Urine culture for GBS anytime during pregnancy
  2.  A prior infant with infant GBS disease

What is the Treatment of Choice, When is it Initiated and for How Long?

  • Penicillin is the treatment of choice and when indicated it is usually started with the onset of labor or with rupture of membranes and is given every 4 hours till the patient delivers. In patients who are allergic to PCN (see below) the antibiotic is again started when the patient is in labor or with rupture of membranes and continued throughout labor. Depending on antibiotic dose frequency ranges from every 4-12 hours.

What is the Treatment of Choice in Patients who are Allergic to Penicillin and are GBS Positive?  

  • The antibiotic of choice depends on the extent of the allergy and results of culture sensitivities (Figure I). If patient has a mild allergy to PCN (nothing more than redness) then Cefazolin is the recommended intra-partum therapy. If the allergy is severe then sensitivities are requested at the time of obtaining the swab.  If the organism is found to be sensitive to both Clindamycin and Erythromycin then Clindamycin is given.  Erythromycin is no longer recommended as a treatment option of GBS due to the growing concern of resistance even in the presence of sensitivity. If GBS is found to be resistant to either erythromycin or Clindamycin then Vancomycin is indicated during labor. In the absence of culture sensitivities and severe allergy Vancomycin is the recommended treatment.

gbs treatment study chart usmle exam

What to do in Laboring Patients When the GBS Status is Unknown?

  • In a laboring if the GBS status is unknown intra-partum therapy is not indicated except in the following scenario’s:
  1.  GBS bacteriuria (positive urine culture) during any trimester of the current pregnancy.
  2.  Preterm Labor (less than 37 weeks of gestation
  3.  Rupture of membranes that occurred 18 hours or more earlier.
  4.  Intra-partum temperature is greater than or equal to 38.0°C (100.4°F).

How Long is a Negative Result Valid?

  • A negative GBS test result is valid for 5 weeks, after which the person needs to be retested. A positive result anytime during pregnancy requires treatment and does not need to be treated.

Clinical Scenarios

  1. A 26 year-old woman, gravida 4, Para 3003 at 39 weeks of gestation presents to the labor and delivery suite in active labor. She is Group B streptococcus (GBS) positive.  Patient reports no allergies. The best antibiotic to use for intra-partum GBS prophylaxis in this scenario is:
    1. Answer: Penicillin
  2. A 26 year-old woman, gravida 4, Para 3003 at 39 weeks of gestation presents to the labor and delivery suite in active labor. She is Group B streptococcus (GBS) positive. Patient reports a severe allergy to penicillin that required admission to the intensive care unit. Her culture and sensitivities show sensitivities to Clindamycin and resistance to Erythromycin. The best antibiotic to use for intra-partum GBS prophylaxis in this scenario is:
    1. Answer: Vancomycin


Note: The correct answer is Vancomycin if the questions was changed to show no sensitivities are available. The answer will be Clindamycin if the questions is changed and is sensitive to both Clindamycin and Erythromycin.

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MD, FACOG, USMLE Step 2 CK Obstetrics and Gynecology Instructor at Becker