Liquid Dosing of Trimethoprim-Sulfamethoxazole for a 21.6 kg Child
For a child weighing 21.6 kg, administer trimethoprim-sulfamethoxazole oral suspension 45 mg trimethoprim/225 mg sulfamethoxazole (7.5 mL of the 6 mg/mL suspension) twice daily for treatment, or once daily for prophylaxis. 1
Dosing Calculation
The child weighs 21.6 kg, which falls into the >15-23 kg (33-51 lb) weight category according to multiple authoritative guidelines 2, 3, 1.
For Treatment Indications:
- Dose: 45 mg trimethoprim twice daily (every 12 hours)
- Volume: 7.5 mL of oral suspension twice daily
- This achieves the recommended 8 mg/kg/day of trimethoprim component divided into 2 doses 2, 1
For Prophylaxis (e.g., PCP prevention):
- Dose: 45 mg trimethoprim once daily
- Volume: 7.5 mL of oral suspension once daily
- For PCP prophylaxis specifically, the standard regimen is 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole 4, 5
Suspension Preparation Details
The commercially available oral suspension has a final concentration of 6 mg/mL trimethoprim and 30 mg/mL sulfamethoxazole 1. If the manufactured suspension is unavailable, retail pharmacies can compound it to the same concentration based on package label instructions 1.
Duration of Therapy
Treatment duration depends on the indication:
- Urinary tract infections/acute otitis media: 10-14 days 1
- Shigellosis: 5 days 1
- Skin and soft tissue infections: 5-7 days 2
- PCP treatment: 14-21 days (requires higher dosing of 15-20 mg/kg/day trimethoprim) 1
- PCP prophylaxis: Continuous, typically three times weekly on consecutive days 4
Important Clinical Considerations
Renal function: The standard dosing above assumes normal renal function. If creatinine clearance is 15-30 mL/min, reduce to half the usual regimen; if <15 mL/min, use is not recommended 1.
Administration: Can be given without regard to meals, though administration with food may improve gastrointestinal tolerability 1.
Monitoring: For prolonged prophylaxis regimens (particularly in HIV-infected children), obtain complete blood counts with differential and platelet count at initiation and monthly intervals to assess for hematologic toxicity 5.
Alternative schedules for prophylaxis: While daily dosing is effective, three-times-weekly dosing on consecutive days (same dose per administration) is also acceptable and may improve tolerability 4, 6.