Bactrim Dosage for 11-Year-Old Girl Weighing 86 lbs
For an 11-year-old girl weighing 86 lbs (39 kg), the appropriate Bactrim dose is 8–12 mg/kg/day of the trimethoprim component, divided into 2 doses given every 12 hours.
Specific Dosing Calculation
Using the child's weight of 39 kg:
- Standard dose range: 8–12 mg/kg/day of trimethoprim
- Daily trimethoprim dose: 312–468 mg/day (39 kg × 8–12 mg/kg)
- Divided into 2 doses: 156–234 mg trimethoprim per dose, given every 12 hours
Practical Tablet Dosing
Since Bactrim double-strength (DS) tablets contain 160 mg trimethoprim and 800 mg sulfamethoxazole:
- 1 DS tablet twice daily provides 320 mg trimethoprim/day (8.2 mg/kg/day) – this is the most appropriate starting dose 1, 2, 3
- 1.5 DS tablets twice daily provides 480 mg trimethoprim/day (12.3 mg/kg/day) – use for more severe infections 1
Indication-Specific Dosing
The exact dose depends on the infection being treated:
For Skin/Soft Tissue Infections (including MRSA)
- 8–12 mg/kg/day trimethoprim in 2 divided doses orally 1, 2
- Practical dose: 1 DS tablet twice daily for this patient
For Urinary Tract Infections
- 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours in 2 divided doses for 10–14 days 3
- Practical dose: 1 DS tablet twice daily
For Pneumocystis Pneumonia (Treatment)
- 75–100 mg/kg sulfamethoxazole and 15–20 mg/kg trimethoprim per 24 hours in 4 divided doses every 6 hours 3
- This patient would need 2 DS tablets every 6 hours (upper limit dosing)
For Pneumocystis Prophylaxis
Important Clinical Considerations
Contraindications: Bactrim is contraindicated in children less than 2 months of age due to risk of kernicterus 3. This 11-year-old patient is well beyond this age restriction.
Renal function: If creatinine clearance is impaired, dose adjustment is necessary 3:
- CrCL >30 mL/min: standard dosing
- CrCL 15–30 mL/min: half the usual dose
- CrCL <15 mL/min: use not recommended
Monitoring: Recent pharmacokinetic data suggests that the standard 8 mg/kg/day trimethoprim dose achieves appropriate exposure matching adult dosing and reaches pharmacodynamic targets for bacteria with MIC ≤0.5 mg/L in >90% of children 6. Higher doses (12–15 mg/kg/day) may be needed for organisms with MIC up to 1 mg/L 6.
Adverse effects: Watch for gastrointestinal symptoms, rash, and rarely Stevens-Johnson syndrome 7, 3. Inadequate weight-based dosing has been associated with clinical failure in cellulitis 8, emphasizing the importance of proper dosing.
Duration: Typical treatment duration is 7–14 days depending on the infection type and clinical response 1, 2, 3.