What is the maximum dose of Bactrim (trimethoprim/sulfamethoxazole) for a 6-year-old patient weighing 56 pounds with potential impaired renal function?

Maximum Bactrim Dose for a 6-Year-Old, 56 lbs (25.4 kg)

For standard infections, the maximum dose is 12 mg/kg/day of trimethoprim (305 mg/day) divided every 12 hours; for severe/life-threatening infections like Pneumocystis pneumonia, the maximum is 20 mg/kg/day of trimethoprim (508 mg/day) divided every 6 hours. 1, 2

Standard Treatment Dosing

• For most infections (UTI, skin/soft tissue, otitis media): The recommended dose is 8-12 mg/kg/day of trimethoprim component, divided every 12 hours 1, 2

  • For this 25.4 kg child: 203-305 mg trimethoprim per day (10-15 mL of suspension twice daily)
  • This corresponds to 40-60 mg/kg/day of sulfamethoxazole 1

• The FDA label specifies the standard pediatric dose as 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours 2

Severe Infection Dosing

• For serious infections (severe MRSA, life-threatening infections): The dose escalates to 10-12 mg/kg/day, or up to 15-20 mg/kg/day of trimethoprim divided every 6-8 hours 1, 2

  • For this 25.4 kg child: 381-508 mg trimethoprim per day (approximately 6-8 mL every 6 hours)

• For documented Pneumocystis jirovecii pneumonia: The FDA recommends 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours, divided every 6 hours for 14-21 days 2

  • For this 25.4 kg child: This equals 381-508 mg trimethoprim per day (approximately 5-7 mL every 6 hours)

Renal Impairment Adjustments

Critical consideration: If this child has impaired renal function, dose reduction is mandatory to prevent toxicity 3, 2

• Creatinine clearance 15-30 mL/min: Reduce dose by 50% 3, 4, 2
• Creatinine clearance <15 mL/min: Reduce dose by 50% or use alternative agent 3, 4, 2
• For treatment dosing with CrCl 10-50 mL/min: Administer 3-5 mg/kg every 12 hours (instead of every 6-8 hours) 3
• For treatment dosing with CrCl <10 mL/min: Administer 3-5 mg/kg every 24 hours 3

Practical Dosing Table for This Patient

Using the FDA guideline table for a child weighing approximately 56 lbs (25.4 kg, closest to 20 kg reference) 2:

• Standard infections: 2 teaspoonfuls (10 mL) every 12 hours
• Severe infections/PCP: 3 teaspoonfuls (15 mL) every 6 hours (upper limit dosing)

Safety Monitoring Requirements

• Obtain complete blood count with differential and platelet count at treatment initiation 1
• Repeat monthly during prolonged therapy to assess for hematologic toxicity 1
• Monitor for thrombocytopenia, which correlates with higher serum TMP levels and longer treatment duration 5
• Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria 3

Important Contraindications and Cautions

• Contraindicated in children <2 months of age 2
• Use with extreme caution in G6PD deficiency due to hemolytic anemia risk 1, 4
• Avoid in severe hepatic impairment 3
• The liquid formulation is preferred for accurate dosing in children <16 kg 1