What are the recommended antibiotic options for Group B Streptococcus (GBS) prophylaxis?

Antibiotic Options for GBS Prophylaxis

First-Line Therapy

Penicillin G is the preferred first-line agent for GBS prophylaxis, administered as 5 million units IV initially, followed by 2.5-3.0 million units IV every 4 hours until delivery. 1, 2

• Penicillin G is preferred over ampicillin due to its narrower spectrum of activity, which reduces selection pressure for antibiotic-resistant organisms 2
• Ampicillin (2 g IV initial dose, then 1 g IV every 4 hours) is an acceptable alternative if penicillin G is unavailable 3, 2
• All GBS isolates worldwide remain universally susceptible to penicillin 4, 5

Penicillin-Allergic Patients: Risk Stratification Required

The choice of alternative antibiotic depends critically on whether the patient is at high risk for anaphylaxis, defined as history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin administration 1, 4

For Patients NOT at High Risk for Anaphylaxis

Cefazolin is the preferred alternative, dosed as 2 g IV initially, then 1 g IV every 8 hours until delivery. 1, 3, 2

• Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients with penicillin allergy 4, 3
• Only first-generation cephalosporins (cefazolin) should be used, as resistance to second-generation agents like cefoxitin has been reported 3
• Many reported penicillin allergies are not true IgE-mediated reactions, and allergy verification should be pursued when possible 3, 6

For Patients at High Risk for Anaphylaxis

The antibiotic choice depends on susceptibility testing results, which MUST be obtained for all high-risk penicillin-allergic patients. 1, 4, 3

If Susceptibility Testing Available:

• Clindamycin 900 mg IV every 8 hours if the GBS isolate is susceptible to both clindamycin and erythromycin 1, 4, 2
• Clindamycin 900 mg IV every 8 hours may be used if susceptible to clindamycin but resistant to erythromycin, ONLY if testing for inducible clindamycin resistance is negative 1, 4
• Vancomycin 1 g IV every 12 hours if the isolate is resistant to clindamycin, demonstrates inducible clindamycin resistance, or if susceptibility is unknown 1, 4, 3, 2

If Susceptibility Testing NOT Available:

• Vancomycin 1 g IV every 12 hours is the recommended default choice when susceptibility results are pending or unavailable 4, 2

Critical Considerations About Resistance Patterns

Clindamycin and Erythromycin Resistance

• Clindamycin resistance rates have increased to 3-15% in the US, with some centers reporting rates as high as 28% 2, 5, 7
• Erythromycin resistance rates reach up to 20-30% in various populations 2, 5, 7
• Erythromycin is no longer recommended for GBS prophylaxis due to increasing resistance 4, 2
• Co-resistance between erythromycin and clindamycin is extremely high at 92% 7
• Only 7.9% of erythromycin-resistant strains remain susceptible to clindamycin 7

Importance of Inducible Resistance Testing

• Testing for inducible clindamycin resistance (D-test) is mandatory for isolates susceptible to clindamycin but resistant to erythromycin 1, 3, 2
• Only 8.6% of clindamycin-resistant GBS demonstrate inducible resistance patterns 7

Common Pitfalls and How to Avoid Them

Pitfall #1: Using Clindamycin Without Susceptibility Testing

• Never use clindamycin for high-risk penicillin-allergic patients without confirmed susceptibility testing 1, 4, 7
• Given resistance rates of 28-30%, empiric clindamycin use risks treatment failure 7
• Default to vancomycin if susceptibility results are unavailable 4

Pitfall #2: Treating GBS Colonization Before Labor

• Antimicrobial agents should NOT be used before the intrapartum period to treat asymptomatic GBS colonization 3
• Antepartum treatment is ineffective in eliminating carriage, does not prevent neonatal disease, and promotes antibiotic resistance 3
• The exception is GBS urinary tract infection (symptomatic or asymptomatic), which requires treatment according to standard UTI protocols 3

Pitfall #3: Inadequate Communication with Laboratory

• Clinicians must explicitly inform laboratories of the need for antimicrobial susceptibility testing in penicillin-allergic patients 1
• Susceptibility testing should be performed on prenatal GBS isolates (obtained at 36 0/7 to 37 6/7 weeks gestation) from all penicillin-allergic women at high risk for anaphylaxis 1, 3, 6

Pitfall #4: Underestimating Anaphylaxis Risk

• The risk of anaphylaxis with penicillin is estimated at 4/10,000 to 4/100,000 recipients 2
• This risk is far outweighed by the benefits of preventing GBS disease, but proper risk stratification remains essential 2
• Anaphylactic reactions can have severe consequences for both mother and child 8

Special Clinical Scenarios

GBS Bacteriuria During Pregnancy

• Women with GBS bacteriuria at any concentration during pregnancy require both treatment of the UTI AND intrapartum prophylaxis during labor 3
• These women should receive intrapartum prophylaxis regardless of whether they were treated earlier in pregnancy 3

Planned Cesarean Delivery

• Women undergoing planned cesarean delivery before labor onset and before membrane rupture do not routinely require intrapartum chemoprophylaxis 3

Screening Timing Update

• The optimal screening window has been updated to 36 0/7 to 37 6/7 weeks gestation (previously 35 0/7 to 37 6/7 weeks) 3, 6