Trimethoprim-Sulfamethoxazole for Urinary Tract Infections
For uncomplicated cystitis in women, prescribe trimethoprim-sulfamethoxazole (TMP-SMX) 160/800 mg (one double-strength tablet) twice daily for 3 days, but only when local E. coli resistance rates are below 20% and the patient has no recent antibiotic exposure. 1, 2
Treatment Regimens by Clinical Scenario
Uncomplicated Cystitis in Women
- First-line therapy: TMP-SMX 160/800 mg twice daily for 3 days achieves clinical cure rates of 90-100% when the organism is susceptible 1, 3, 4
- Alternative first-line agents (preferred when TMP-SMX resistance >20%):
Uncomplicated Cystitis in Men
- Standard regimen: TMP-SMX 160/800 mg twice daily for 7 days (longer duration than women) 1, 2
- Fluoroquinolones may be prescribed according to local susceptibility testing 1
Uncomplicated Pyelonephritis
- TMP-SMX regimen: 160/800 mg twice daily for 14 days, but only after culture confirms susceptibility 1, 3
- Fluoroquinolone alternative: 5-7 days when susceptibility is known 1
- Critical caveat: TMP-SMX should not be used empirically for pyelonephritis without culture and susceptibility testing due to high failure rates with resistant organisms 1
When NOT to Use TMP-SMX
Resistance Considerations
- Do not use empirically when local E. coli resistance rates exceed 20% 1, 3
- In areas with high TMP-SMX resistance, empiric use results in 58% microbiologic failure (only 42% cure rate vs 86% with susceptible organisms) 5
- Always obtain culture before using TMP-SMX for pyelonephritis, as empiric use without susceptibility data leads to treatment failure 1
Pregnancy Contraindications
- Avoid in first trimester due to teratogenic concerns 1
- Contraindicated in last trimester due to risk of kernicterus 1
Patient-Specific Contraindications
- Contraindicated in children <2 months of age 2, 6
- Avoid in patients with recent antibiotic exposure (increases resistance risk) 1
Renal Dosing Adjustments
- CrCl >30 mL/min: Standard dosing 2, 6
- CrCl 15-30 mL/min: Reduce to half the usual regimen 2, 6
- CrCl <15 mL/min: Use not recommended 2, 6
Critical Diagnostic Requirements Before Treatment
Confirm True Infection (Not Colonization)
- Required criteria: BOTH pyuria (≥10 WBCs/HPF or positive leukocyte esterase) AND acute urinary symptoms (dysuria, frequency, urgency, fever >38.3°C, or gross hematuria) 7
- Do not treat asymptomatic bacteriuria with pyuria—this represents colonization, not infection, and treatment provides no benefit while increasing resistance 7
Obtain Proper Specimen
- Use midstream clean-catch or catheterization to avoid contamination 7
- High epithelial cell counts indicate contamination and invalidate results 7
Common Pitfalls to Avoid
Overtreatment of Asymptomatic Bacteriuria
- Asymptomatic bacteriuria occurs in 15-50% of elderly patients and should never be treated (except in pregnancy or before urologic procedures with mucosal bleeding) 7
- Pyuria alone without symptoms does not justify treatment 7
Empiric Use in High-Resistance Areas
- When local resistance exceeds 20%, choose nitrofurantoin or fosfomycin instead 1, 3
- For pyelonephritis, never use TMP-SMX empirically—always wait for susceptibility results 1
Inadequate Treatment Duration
- Women with cystitis: 3 days is sufficient 1, 2
- Men with cystitis: 7 days required 1, 2
- Pyelonephritis: 14 days mandatory 1, 3
Fluoroquinolone Overuse
- Reserve fluoroquinolones for complicated infections or documented resistance to first-line agents 1, 4
- High propensity for adverse effects makes them inappropriate for empiric uncomplicated cystitis 1