What is the recommended dosage of penicillin (Penicillin G) for group B streptococcus (GBS)?

Penicillin G Dosing for Group B Streptococcus

For intrapartum GBS prophylaxis, administer penicillin G 5 million units IV as an initial loading dose, followed by 2.5-3.0 million units IV every 4 hours until delivery. 1, 2

Dosing Regimen Details

Standard Protocol

• Loading dose: 5 million units IV initially 1, 2
• Maintenance dose: 2.5-3.0 million units IV every 4 hours until delivery 1, 2
• The range of 2.5-3.0 million units is specifically designed to achieve adequate drug levels in fetal circulation and amniotic fluid while avoiding neurotoxicity 1
• Choose the dose within this range based on which penicillin G formulations are readily available in your pharmacy to reduce the need for special preparation 1

Route of Administration

• Intravenous administration is the only acceptable route for GBS prophylaxis, as it achieves the necessary high concentrations in maternal and fetal circulation 2

Pharmacokinetic Rationale

Timing Considerations

• Optimal prophylaxis requires at least 4 hours of antibiotic administration before delivery to maximize effectiveness 1
• However, even shorter durations provide benefit: fetal penicillin G levels exceed the minimum inhibitory concentration (MIC) for GBS by 10-179 fold, even with brief exposure 3
• Penicillin G levels increase linearly for the first hour after administration, then decrease rapidly according to a power-decay model 3
• The drug does not accumulate in cord blood with maintenance dosing and returns to baseline after each 4-hour interval 3

Dosing Interval Justification

• The 4-hour dosing interval is critical because average serum penicillin concentration drops to approximately 12 mcg/mL after 4 hours, necessitating redosing to maintain anti-GBS activity 4
• More frequent dosing does not increase antimicrobial activity 4

Alternative First-Line Agent

Ampicillin

• Dosing: 2 g IV initial dose, then 1 g IV every 4 hours until delivery 2, 5
• Ampicillin is an acceptable alternative, though penicillin G is preferred due to its narrower spectrum of activity, which reduces selection pressure for resistant organisms 2

Penicillin-Allergic Patients

Risk Stratification Required

• High-risk for anaphylaxis is defined as history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin administration 1, 2

For Patients NOT at High Risk for Anaphylaxis

• Cefazolin: 2 g IV initial dose, then 1 g IV every 8 hours until delivery 2, 5, 6
• Approximately 10% of penicillin-allergic patients will cross-react with cephalosporins, making risk assessment essential 5, 6

For Patients at High Risk for Anaphylaxis

• Clindamycin: 900 mg IV every 8 hours until delivery (only if GBS isolate is susceptible to both clindamycin and erythromycin) 2, 5, 6
• Vancomycin: 1 g IV every 12 hours until delivery (if susceptibility testing unavailable or isolate is resistant to clindamycin) 2, 5, 6
• Susceptibility testing is mandatory for penicillin-allergic patients at high risk for anaphylaxis, as clindamycin resistance rates have increased to 3-15% in the US 2
• Testing for inducible clindamycin resistance is necessary for isolates susceptible to clindamycin but resistant to erythromycin 1, 2

Critical Clinical Pitfalls

What NOT to Do

• Never use erythromycin for GBS prophylaxis in penicillin-allergic patients at high risk for anaphylaxis, as it is no longer acceptable due to resistance rates up to 20.2% 1, 2
• Do not treat GBS colonization before the intrapartum period with antimicrobials, as this is ineffective in eliminating carriage and may promote resistance 5
• Do not delay medically necessary obstetric procedures to achieve 4 hours of prophylaxis, though timing procedures to allow adequate prophylaxis is preferred when possible 1

Special Populations

• Women with GBS bacteriuria at any concentration during pregnancy require both treatment of the UTI and intrapartum prophylaxis during labor, regardless of whether they were treated earlier in pregnancy 5
• The risk of anaphylaxis with penicillin (4/10,000 to 4/100,000 recipients) is far outweighed by the benefits of preventing GBS disease 2