What is the recommended dose of ampicillin (Ampicillin) for a urinary tract infection (UTI) caused by Group B streptococcus (GBS)?

Ampicillin Dosing for Group B Streptococcal UTI

For Group B streptococcal urinary tract infections, use ampicillin 2 g IV initially, followed by 1 g IV every 4-6 hours until symptoms resolve, typically for 7-14 days depending on severity. 1

Treatment Approach Based on Patient Population

For Pregnant Women with GBS UTI

Immediate treatment of the acute UTI is mandatory, followed by intrapartum antibiotic prophylaxis during labor regardless of when the UTI was treated. 1

• Acute UTI treatment: Ampicillin 2 g IV initially, then 1 g IV every 4 hours for symptomatic infection 1
• GBS bacteriuria at any concentration during pregnancy indicates heavy genital tract colonization and significantly increases risk of early-onset neonatal GBS disease 1
• Treating the UTI does NOT eliminate GBS colonization from the genitourinary tract—recolonization after antibiotics is typical, which is why intrapartum prophylaxis remains mandatory 1

Intrapartum prophylaxis during labor (separate from UTI treatment):

• Ampicillin 2 g IV loading dose, then 1 g IV every 4 hours until delivery 1
• Alternative: Penicillin G 5 million units IV initially, then 2.5 million units IV every 4 hours (preferred due to narrower spectrum) 1
• Intrapartum prophylaxis administered ≥4 hours before delivery is 78% effective in preventing early-onset neonatal GBS disease 1

For Non-Pregnant Adults with GBS UTI

• Standard dosing: Ampicillin 2 g IV initially, then 1 g IV every 6 hours for 7-14 days depending on severity 2
• For uncomplicated UTI in non-pregnant patients, oral amoxicillin 500 mg three times daily for 7 days is an acceptable alternative 3
• GBS shows 96% susceptibility to ampicillin, making it highly effective 4

Alternative Regimens for Penicillin Allergy

Non-Severe Penicillin Allergy

• Cefazolin 2 g IV initially, then 1 g IV every 8 hours until completion of therapy 3, 1
• Approximately 10% of persons with penicillin allergy have cross-reactivity to cephalosporins, so this is only appropriate for non-severe allergies 1

Severe Penicillin Allergy (High Risk for Anaphylaxis)

• Clindamycin 900 mg IV every 8 hours if the isolate is confirmed susceptible 3, 1
• Vancomycin 1 g IV every 12 hours if susceptibility testing unavailable or isolate resistant to clindamycin 3, 1
• Susceptibility testing is essential for penicillin-allergic patients at high risk for anaphylaxis 1

Oral Options for Uncomplicated UTI in Penicillin-Allergic Patients

• Nitrofurantoin 100 mg orally every 6 hours (GBS shows 95.5% susceptibility) 3, 4
• Cephalexin for non-pregnant patients with non-severe allergy 3

Critical Clinical Pitfalls to Avoid

Do NOT treat asymptomatic GBS colonization outside of pregnancy or the intrapartum period. 3, 1

• Treatment of asymptomatic colonization is ineffective in eliminating carriage and promotes antibiotic resistance 3, 1
• The exception is GBS bacteriuria during pregnancy, which requires both immediate treatment and intrapartum prophylaxis 1

Do NOT use macrolides (erythromycin) or tetracyclines as first-line agents. 4, 5

• GBS shows 81.6% resistance to tetracycline and 68.9% resistance to co-trimoxazole 4
• Erythromycin resistance rates range from 16-45% depending on geographic location 5

Do NOT underdose or prematurely discontinue therapy. 3

• Incomplete treatment leads to treatment failure or recurrence 3
• Complete the full prescribed course even if symptoms resolve early 1

Monitoring and Follow-Up

• No routine follow-up urine culture is required if symptoms resolve completely 3
• If symptoms persist after completion of treatment, obtain urine culture to check for treatment failure or reinfection 3
• For pregnant women, ensure documentation of GBS bacteriuria in prenatal records to guarantee intrapartum prophylaxis is administered during labor 1