Ampicillin Dosage for Uncomplicated UTI in Women
For uncomplicated urinary tract infections in women, ampicillin should be dosed at 500 mg orally four times daily (q.i.d.) in equally spaced doses. 1
Recommended Dosing Regimen
- Ampicillin 500 mg orally four times daily is the FDA-approved dosage for genitourinary tract infections in adult women 1
- Treatment should be continued for a minimum of 48 to 72 hours after the patient becomes asymptomatic or evidence of bacterial eradication has been obtained 1
- For optimal absorption, ampicillin should be administered at least one-half hour before or two hours after meals 1
Important Considerations
- Ampicillin should not be used for empirical treatment of uncomplicated UTIs due to relatively poor efficacy and high prevalence of antimicrobial resistance worldwide 2
- The Infectious Diseases Society of America (IDSA) specifically recommends against using amoxicillin or ampicillin for empirical treatment of uncomplicated UTIs (Grade A-III recommendation) 2
- A urine culture and susceptibility testing should be performed to confirm that the causative organism is susceptible to ampicillin before initiating treatment 2
First-Line Alternatives
If the patient has not yet had susceptibility testing or if ampicillin resistance is suspected, consider these first-line agents instead:
- Nitrofurantoin 100 mg orally four times daily for uncomplicated UTIs 2
- Trimethoprim-sulfamethoxazole 160/800 mg (double-strength tablet) twice daily for 3 days if local resistance rates are below 20% 2
- Fosfomycin 3 g orally as a single dose 2
Special Situations
- For VRE (Vancomycin-Resistant Enterococci) causing uncomplicated UTIs, high-dose ampicillin (18-30 g IV daily in divided doses) or amoxicillin 500 mg IV or PO every 8 hours is recommended 2
- For complicated UTIs requiring hospitalization, parenteral ampicillin may be used as part of combination therapy with an aminoglycoside 2
Monitoring and Follow-up
- In chronic urinary infections, frequent bacteriologic and clinical appraisal is necessary during therapy and may be necessary for several months afterward 1
- Stubborn infections may require treatment for several weeks 1
- Follow-up urine cultures should be obtained after completion of therapy to confirm eradication of the infection 2
Pitfalls to Avoid
- Using ampicillin empirically without susceptibility testing can lead to treatment failure due to high resistance rates 2
- Underdosing ampicillin (smaller doses than indicated) should not be used as this may lead to treatment failure and promote resistance 1
- Not completing the full course of therapy even if symptoms resolve quickly 1
- Not spacing doses equally throughout the day, which can lead to suboptimal drug levels 1