What alternative antibiotic should be used for a patient with a urinary tract infection (UTI) caused by Escherichia coli, which is resistant to ampicillin, and was initially prescribed Keflex (Cephalexin)?

Treatment for E. coli UTI with Ampicillin Resistance

For a patient with a UTI caused by ampicillin-resistant E. coli who was initially prescribed Keflex (cephalexin), nitrofurantoin is the most appropriate alternative antibiotic due to its high efficacy against E. coli and low resistance rates.

Evaluating the Current Situation

The patient has a confirmed UTI with:

• Culture growing >100,000 CFU/ml of E. coli
• Confirmed ampicillin resistance
• Initial prescription of Keflex (cephalexin)

First-Line Treatment Options

Nitrofurantoin

• Highest recommendation based on current evidence
• 94% susceptibility rate for E. coli urinary isolates 1
• Extremely low resistance rates (0.9-1.1%) across multiple studies 1, 2
• Recommended as first-line therapy by current guidelines 3
• Dosing: 100mg twice daily for 5 days for uncomplicated UTI

Fosfomycin

• Alternative first-line option with low resistance rates (4.3%) 1
• Single 3g dose for uncomplicated UTI 3
• Good option for patients who cannot tolerate multiple-day regimens

Trimethoprim-Sulfamethoxazole (TMP-SMX)

• Not recommended due to high resistance rates (20-46.8%) 1, 4
• Should only be used if susceptibility is confirmed

Second-Line Options

Fluoroquinolones (e.g., Ciprofloxacin)

• Effective against E. coli UTIs but should be reserved for complicated cases 5
• FDA-approved for E. coli UTIs 5
• Dosing: 250-500mg twice daily for 3-7 days depending on severity
• Resistance concerns (5.1-49.9%) make this a second-line option 1, 2

Cephalosporins

• Since Keflex (cephalexin) was initially prescribed, consider other cephalosporins if susceptibility is confirmed
• Resistance rates to cephalexin can be high (30.9-38.8%) 1, 4
• Third-generation cephalosporins may be more effective

Special Considerations

• Extended-Spectrum Beta-Lactamase (ESBL) producing E. coli: If suspected, nitrofurantoin remains effective for lower UTIs 6
• Complicated UTI: Consider parenteral therapy with aminoglycosides or carbapenems if signs of systemic infection 7
• Recurrent UTI: May be an independent risk factor for TMP-SMX resistance 4

Treatment Algorithm

1. First choice: Nitrofurantoin 100mg twice daily for 5 days
2. Alternative first-line: Fosfomycin 3g single dose
3. If contraindications to above: Fluoroquinolone (e.g., ciprofloxacin 500mg twice daily for 3-7 days)
4. For complicated UTI or systemic symptoms: Consider parenteral therapy and infectious disease consultation

Monitoring and Follow-up

• Symptoms should improve within 48-72 hours
• Consider follow-up urine culture in complicated cases or treatment failures
• No routine post-treatment cultures needed for uncomplicated UTIs with symptom resolution

Pitfalls to Avoid

• Continuing Keflex despite high resistance rates in ampicillin-resistant E. coli
• Using TMP-SMX empirically given high resistance rates
• Overusing fluoroquinolones for uncomplicated UTIs
• Failing to adjust therapy based on culture and susceptibility results

Nitrofurantoin offers the best balance of efficacy, safety, and low resistance for treating this patient's ampicillin-resistant E. coli UTI.