Bactrim DS for Uncomplicated UTI
Bactrim DS (sulfamethoxazole-trimethoprim 160/800 mg) is highly effective for uncomplicated UTIs when local E. coli resistance is below 20%, achieving 90-100% clinical cure rates with a 3-day course for women, but should be avoided as empiric therapy when resistance exceeds this threshold. 1
Treatment Regimen
For women with uncomplicated cystitis: Bactrim DS one tablet twice daily for 3 days achieves clinical cure rates of 90-100% and bacterial eradication rates of 91-100% when organisms are susceptible 1
For men with UTI: A longer duration of 7 days is required at the same dose (one double-strength tablet twice daily), as the 3-day regimen studied in women is inadequate 1, 2
For uncomplicated pyelonephritis: 14 days of therapy is needed, but only after confirming susceptibility 1
Critical Resistance Threshold
The key decision point is local E. coli resistance rates:
Use Bactrim DS empirically when local resistance is <20% - At this threshold, cure rates remain 84% for susceptible organisms 1
Avoid Bactrim DS empirically when local resistance exceeds 20% - Treatment failure rates increase dramatically, with clinical cure dropping to only 41-54% when organisms are resistant 1, 3
A landmark 2002 study demonstrated that in high-resistance areas, microbiologic cure fell to just 42% in the TMP-SMX-resistant group versus 86% in the susceptible group 3
When to Avoid Empiric Use
Do not use Bactrim DS empirically if the patient has:
- Used trimethoprim-sulfamethoxazole in the preceding 3-6 months 1
- Traveled outside the United States in the preceding 3-6 months 1, 2
- Pregnancy, especially in the last trimester 1, 2
- Marked hepatic damage 1
Alternative First-Line Agents
When Bactrim cannot be used due to resistance or contraindications, choose:
Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days (90% clinical cure, 92% bacterial cure) 1, 2
Fosfomycin trometamol 3g single dose (equally effective with single-dose convenience) 1, 2
Pivmecillinam 400 mg three times daily for 3-5 days (resistance rates generally <10%) 1, 2
Common Pitfalls to Avoid
Do not use hospital antibiograms to guide outpatient therapy - They overestimate community resistance rates; local outpatient surveillance data is more accurate 1
Do not use the 3-day regimen for men - This is a critical error that leads to treatment failure 2
Do not ignore recent antibiotic exposure - Prior TMP-SMX use within 3-6 months independently predicts resistance 1
Do not use amoxicillin or ampicillin as alternatives - These have very high worldwide resistance rates and poor efficacy 2
Side Effects
Common adverse effects include rash, urticaria, nausea, vomiting, and hematologic abnormalities (thrombocytopenia, neutropenia) 1
FDA-Approved Indication
The FDA approves sulfamethoxazole-trimethoprim for treatment of UTIs due to susceptible strains of E. coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis, and Proteus vulgaris 4, 5