What is the recommended Bactrim double‑strength (sulfamethoxazole 800 mg + trimethoprim 160 mg) dosing for uncomplicated urinary‑tract infection in a non‑pregnant adult, and how should the dose be adjusted for impaired renal function?

Bactrim DS Dosing for Uncomplicated UTI

Standard Dosing for Women

For uncomplicated cystitis in women, prescribe Bactrim DS (sulfamethoxazole 800 mg/trimethoprim 160 mg) one tablet twice daily for 3 days, but only if local E. coli resistance rates are below 20% or if susceptibility is confirmed. 1, 2

• The 3-day regimen achieves clinical cure rates of 90-100% when organisms are susceptible 2
• Bacterial eradication rates range from 91-100% for susceptible pathogens 2
• However, efficacy plummets to only 41-54% when the organism is resistant, making treatment failure the expected outcome 2, 3

Critical Resistance Threshold

Do not use Bactrim empirically when local E. coli resistance exceeds 20%, as treatment failures outweigh benefits at this threshold 1, 2

• This 20% cutoff is based on expert opinion derived from clinical, in vitro, and mathematical modeling studies 1
• In one study from a high-resistance area, microbiologic cure was achieved in only 42% of patients with TMP-SMX-resistant organisms versus 86% with susceptible organisms 3

Risk Factors Predicting Resistance

Avoid Bactrim empirically in patients with:

• Use of trimethoprim-sulfamethoxazole in the preceding 3-6 months 2
• Travel outside the United States in the preceding 3-6 months 2

Dosing for Men

For men with uncomplicated UTI, prescribe Bactrim DS one tablet twice daily for 7 days (not 3 days) 2

Renal Dose Adjustment

When renal function is impaired, adjust dosing as follows 4:

• CrCl >30 mL/min: Standard dosing (one DS tablet twice daily)
• CrCl 15-30 mL/min: Half the usual regimen (one DS tablet once daily)
• CrCl <15 mL/min: Use not recommended 4

Pyelonephritis Dosing

For uncomplicated pyelonephritis, prescribe Bactrim DS one tablet twice daily for 14 days, but only after confirming susceptibility with urine culture 2

First-Line Alternatives When Bactrim Cannot Be Used

When local resistance exceeds 20% or other contraindications exist, use these alternatives:

• Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days: Achieves 90% clinical cure and 92% bacterial cure rates with minimal resistance 1, 2
• Fosfomycin trometamol 3 g single dose: Convenient single-dose therapy with minimal resistance, though slightly inferior efficacy compared to standard regimens 1, 2
• Fluoroquinolones (ciprofloxacin 250 mg twice daily for 3 days): Highly effective (93-97% eradication) but should be reserved for situations where first-line agents cannot be used due to collateral damage concerns 1, 2

Important Contraindications

Avoid Bactrim in:

• Pregnant women, especially in the last trimester 2
• Patients with marked hepatic damage 2
• Patients at risk for hematologic abnormalities (thrombocytopenia, neutropenia) 2

Common Pitfalls

• Do not rely on hospital antibiograms for outpatient UTI treatment decisions, as they often overestimate community resistance rates; use local outpatient surveillance data instead 2
• Do not treat asymptomatic bacteriuria with antibiotics 2
• Do not prescribe longer than 3 days for uncomplicated cystitis in women, as each additional day carries a 5% increased risk for antibiotic-associated adverse events without additional benefits 2