Bactrim Dosing for a 35 lb Pediatric Female
For a 35 lb (15.9 kg) female child, the recommended dose of Bactrim is 8 mg/kg/day of the trimethoprim component divided into two doses every 12 hours, which equals approximately 64 mg of trimethoprim twice daily (or 3.2 mL of the standard suspension twice daily). 1
Weight-Based Dosing Calculation
- Standard pediatric dosing: 8 mg/kg/day of trimethoprim (or 40 mg/kg/day of sulfamethoxazole) divided into two doses every 12 hours 1
- For 15.9 kg (35 lb) child: This equals approximately 127 mg trimethoprim per day total, or 64 mg per dose 2, 3
- Using standard suspension (40 mg TMP/200 mg SMX per 5 mL): Give 3.2 mL (approximately 3 mL) twice daily 3
Indication-Specific Dosing
For Urinary Tract Infections or Acute Otitis Media
- Dose: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided every 12 hours for 10 days 1
- For 35 lb child: According to FDA dosing tables, this falls into the 22-44 lb weight range, which corresponds to 1 tablet (400 mg/80 mg) every 12 hours 1
- Liquid formulation: 3-3.5 mL of suspension (40 mg TMP/200 mg SMX per 5 mL) twice daily 3
For Skin/Soft Tissue Infections (Impetigo)
- Dose: 8-12 mg/kg/day of trimethoprim component divided into two doses for 7 days 3
- For this weight: 2.5-3.5 mL of suspension twice daily provides appropriate coverage 3
For Pneumocystis Prophylaxis
- Dose: 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole, given three consecutive days per week 2
- Alternative: 8 mg/kg/day trimethoprim divided into two doses 2
Important Clinical Considerations
Age restriction: Bactrim should not be used in children less than 2 months of age due to risk of kernicterus 1
Duration varies by indication:
Monitoring requirements:
- For prophylactic use: Complete blood counts with differential and platelet count at initiation and monthly thereafter 2
- Watch for common side effects including rash, gastrointestinal symptoms, and hematologic abnormalities 2, 3
Hydration: Ensure adequate fluid intake during treatment to prevent crystalluria 3
Renal adjustment: Not typically needed at this age unless underlying renal disease is present; dose adjustment required only when creatinine clearance <30 mL/min 1
Practical Prescribing
For standard treatment (UTI/otitis media):
- Trimethoprim-Sulfamethoxazole suspension (40 mg TMP/200 mg SMX per 5 mL)
- Dose: 3 mL by mouth twice daily for 10 days
- Quantity: 60 mL 3
Common pitfall: Using adult dosing or tablet formulations in young children can lead to dosing errors; liquid suspension allows for more accurate weight-based dosing in this age group 3