Bactrim Dosing for Pediatric UTI
For uncomplicated urinary tract infections in children over 2 months of age, administer trimethoprim-sulfamethoxazole at 6–12 mg/kg/day of the trimethoprim component (30–60 mg/kg/day sulfamethoxazole) divided into 2 doses every 12 hours for 7–14 days. 1
Standard Dosing Regimen
The American Academy of Pediatrics recommends 8 mg/kg/day trimethoprim with 40 mg/kg/day sulfamethoxazole, divided every 12 hours, for most pediatric infections including UTI. 2 This dosing achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children. 2
Practical Weight-Based Dosing
The FDA label provides specific tablet-based dosing for children: 3
- 22 lb (10 kg): 1 single-strength tablet (80 mg TMP/400 mg SMX) every 12 hours
- 44 lb (20 kg): 1 single-strength tablet every 12 hours
- 66 lb (30 kg): 1½ single-strength tablets every 12 hours
- 88 lb (40 kg): 2 single-strength tablets OR 1 double-strength tablet every 12 hours
Liquid Formulation for Younger Children
Use the oral suspension (40 mg trimethoprim per 5 mL) for accurate dosing in children weighing <16 kg. 2 For example, a 31 kg child requiring 10 mg/kg/day would need approximately 156 mg per dose, which equals roughly 19.5 mL of suspension every 12 hours. 2
Treatment Duration
The total course should be 7–14 days for pediatric UTI. 1, 3 Evidence demonstrates that shorter courses of 1–3 days are inferior to the recommended 7–14 day range, particularly for febrile UTIs. 1 Single-dose regimens, while effective at clearing initial bacteriuria, carry a 23% risk of recurrent infection within 10 days compared to only 2% with standard 7-day courses. 4
Route of Administration
Most children with UTI can be treated with oral antibiotics. 1, 5
Parenteral therapy is indicated when the child: 1, 5
- Appears clinically "toxic"
- Cannot retain oral intake including medications
- Has compliance concerns with oral administration
For parenteral therapy, use trimethoprim 15–20 mg/kg/day divided every 6 hours, then transition to oral dosing once the child can tolerate it. 2
Maximum Daily Dose
The maximum daily dose is 320 mg trimethoprim/1600 mg sulfamethoxazole (equivalent to 2 double-strength tablets per day). 3
Important Clinical Considerations
Renal Impairment Adjustments
For creatinine clearance 15–30 mL/min, reduce the dose by 50%. 2, 3 For CrCl <15 mL/min, either reduce by 50% or select an alternative agent. 2, 3
Monitoring Requirements
Obtain a complete blood count with differential and platelet count at treatment initiation, and repeat monthly during prolonged therapy to assess for hematologic toxicity. 2, 1
Hydration
Ensure adequate fluid intake (at least 1.5 L daily in older children, proportionally less in younger children) to prevent crystalluria. 2
Common Pitfalls to Avoid
- Do not use nitrofurantoin for febrile UTIs or suspected pyelonephritis in infants, as it does not achieve adequate serum and parenchymal concentrations. 1, 5
- Do not treat asymptomatic bacteriuria, as treatment may be harmful. 5
- Do not use Bactrim in children under 2 months of age. 3
- Screen for G6PD deficiency before initiating therapy due to hemolytic anemia risk. 2
Antibiotic Selection Guidance
Base empiric therapy on local antimicrobial susceptibility patterns and adjust based on culture results when available. 1, 5 Bactrim remains a first-line option for lower UTI (cystitis) alongside amoxicillin-clavulanic acid. 5
Adverse Reaction Management
For mild rash: Temporarily discontinue and restart once resolved. 2
For severe reactions (Stevens-Johnson syndrome, anaphylaxis, urticarial rash with hypotension): Permanently discontinue Bactrim. 2, 1