What dose of trimethoprim‑sulfamethoxazole suspension should be given to a 21.6 kg child for a urinary tract infection?

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Last updated: September 28, 2026 • View editorial policy

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Dosing Trimethoprim-Sulfamethoxazole Suspension for a 21.6 kg Child with UTI

For a 21.6 kg child with a urinary tract infection, administer trimethoprim-sulfamethoxazole suspension at a dose of 8 mg/kg/day of trimethoprim (173 mg) and 40 mg/kg/day of sulfamethoxazole (864 mg), divided into two doses given every 12 hours for 7-14 days.

Dose Calculation

The FDA-approved dosing for pediatric UTIs is 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours 1. This is corroborated by pediatric UTI guidelines 2.

For your 21.6 kg patient:

  • Trimethoprim: 8 mg/kg × 21.6 kg = 172.8 mg per day (86.4 mg per dose, twice daily)
  • Sulfamethoxazole: 40 mg/kg × 21.6 kg = 864 mg per day (432 mg per dose, twice daily)

Practical Suspension Dosing

Standard TMP-SMX suspension contains 40 mg trimethoprim/200 mg sulfamethoxazole per 5 mL. Based on the FDA dosing table for children, a 22 kg (approximately 44 lb) child should receive 1 tablet equivalent every 12 hours 1.

This translates to approximately 10 mL of suspension twice daily (which provides 80 mg trimethoprim/400 mg sulfamethoxazole per dose, or 160 mg/800 mg daily total).

Treatment Duration

Treat for 7-14 days. The 2011 AAP Pediatrics guideline recommends 7-14 days for febrile UTIs in children 2-24 months 2. While this guideline specifically addresses younger children, the duration principle applies broadly to pediatric UTIs. Seven days may be sufficient for uncomplicated cases, while 14 days is appropriate for more severe presentations or complicated infections.

Key Clinical Considerations

  • Verify the child is ≥2 months old - TMP-SMX is not recommended for infants under 2 months 1
  • Check local resistance patterns - TMP-SMX should only be used if local E. coli resistance is <20% 2. Geographic variability in resistance is substantial, and empiric TMP-SMX may be inappropriate in high-resistance areas
  • Oral vs. parenteral route: For non-toxic appearing children who can tolerate oral intake, oral therapy is equally effective as parenteral 2
  • Do not use nitrofurantoin for febrile UTIs or suspected pyelonephritis, as it doesn't achieve adequate tissue concentrations 2

Common Pitfalls to Avoid

  • Underdosing: Ensure you're calculating based on the trimethoprim component (8 mg/kg/day), not just using arbitrary "teaspoons"
  • Wrong formulation: Don't confuse prophylactic dosing (much lower, once daily) with treatment dosing (twice daily at full dose)
  • Renal impairment: Reduce dose by 50% if creatinine clearance is 15-30 mL/min; avoid if <15 mL/min 1
  • Monitoring in infants: Be aware of rare neutropenia risk, particularly in young infants, though this typically resolves spontaneously 3

References

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.