What is the recommended dosing of Bactrim (trimethoprim/sulfamethoxazole) for a pediatric patient weighing 50 kg with normal renal function?

Bactrim Dosing for a 50 kg Pediatric Patient

For a 50 kg pediatric patient with normal renal function, administer Bactrim at 8-12 mg/kg/day of the trimethoprim component (400-600 mg trimethoprim daily), divided into two doses every 12 hours, which equals 200-300 mg trimethoprim per dose or 1-1.5 double-strength tablets twice daily. 1, 2

Weight-Based Calculation

• Standard dosing: 8-12 mg/kg/day of trimethoprim component divided every 12 hours 1, 2
• For this 50 kg patient:
  • Lower range: 8 mg/kg/day × 50 kg = 400 mg trimethoprim daily (200 mg every 12 hours)
  • Upper range: 12 mg/kg/day × 50 kg = 600 mg trimethoprim daily (300 mg every 12 hours)
• Practical tablet dosing: 1 to 1.5 double-strength tablets (160/800 mg) every 12 hours 3

Indication-Specific Adjustments

• Mild-to-moderate infections (UTI, skin/soft tissue): Use 8-10 mg/kg/day trimethoprim component divided every 12 hours 4, 1
• Serious infections (severe MRSA): Use 10-12 mg/kg/day trimethoprim component, or up to 15-20 mg/kg/day divided every 6-8 hours for life-threatening infections 4, 2
• Pneumocystis jiroveci pneumonia: Requires higher dosing at 15-20 mg/kg/day trimethoprim component divided every 6 hours 3

Formulation Selection

• At 50 kg, tablet formulation is appropriate rather than liquid suspension 2
• Each double-strength tablet contains 160 mg trimethoprim and 800 mg sulfamethoxazole 3
• Each single-strength tablet contains 80 mg trimethoprim and 400 mg sulfamethoxazole 3

Pharmacokinetic Considerations

• The standard 8-12 mg/kg/day dosing achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children and matches adult exposure 5
• For organisms with MIC up to 1 mg/L, higher dosing (12-15 mg/kg/day) may be needed in younger children 5
• At 50 kg, this patient's clearance approximates adult values, supporting standard adult-equivalent dosing 5

Critical Safety Monitoring

• Obtain baseline CBC with differential and platelet count before initiating therapy 2
• Repeat CBC monthly during prolonged therapy (>14 days) to monitor for thrombocytopenia and other hematologic toxicity 2
• Monitor for hypersensitivity reactions including Stevens-Johnson syndrome, particularly in the first 2 weeks 1, 6
• Verify no contraindications: hypersensitivity to sulfonamides/trimethoprim, megaloblastic anemia, age <2 months 1, 6, 3

Renal Function Considerations

• With normal renal function (CrCl >30 mL/min), use standard dosing without adjustment 3
• If CrCl 15-30 mL/min, reduce dose by 50% 3
• If CrCl <15 mL/min, Bactrim is not recommended 3

Common Pitfalls to Avoid

• Do not use in children <2 months of age due to risk of kernicterus 3
• Avoid underdosing by calculating based on trimethoprim component, not the combination weight 1, 2
• Do not use once-daily dosing for serious infections; twice-daily dosing is required for adequate exposure 4, 5
• Be cautious with concurrent warfarin, methotrexate, or oral hypoglycemics due to significant drug interactions 2