Bactrim Dosing for a 21.3 kg, 6-Year-Old Child
For this 21.3 kg child, administer trimethoprim-sulfamethoxazole at 8–12 mg/kg/day of the trimethoprim component (170–255 mg/day total trimethoprim), divided into two doses given every 12 hours, which translates to approximately 1 to 1½ single-strength tablets (80 mg TMP/400 mg SMX) twice daily depending on infection severity. 1
Standard Dosing Framework
The American Academy of Pediatrics establishes the foundation for pediatric TMP-SMX dosing at 8–12 mg/kg/day of trimethoprim and 40–60 mg/kg/day of sulfamethoxazole, divided every 12 hours for children over 2 months of age. 1, 2 The FDA label confirms this approach, specifying 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours in two divided doses for urinary tract infections and acute otitis media. 3
Practical Calculation for This Patient
For a 21.3 kg child:
Standard dosing (8 mg/kg/day TMP): 170 mg/day ÷ 2 = 85 mg per dose = approximately 1 single-strength tablet (80 mg TMP) twice daily 1, 3
Higher-end dosing (12 mg/kg/day TMP): 255 mg/day ÷ 2 = 127.5 mg per dose = approximately 1½ single-strength tablets (120 mg TMP) twice daily 1
The FDA weight-based table supports 1 single-strength tablet every 12 hours for children weighing 20 kg, making this the most straightforward starting point for routine infections. 3
Indication-Specific Adjustments
For mild-to-moderate infections (uncomplicated UTI, skin/soft tissue infections), use the standard 8–10 mg/kg/day range, which corresponds to 1 single-strength tablet twice daily for this weight. 1
For serious infections (severe MRSA, complicated infections), the Infectious Diseases Society of America recommends escalating to 10–12 mg/kg/day or even 15–20 mg/kg/day divided every 6–8 hours for life-threatening cases. 1 For this child, serious infections would require 1½ single-strength tablets twice daily at minimum.
Population pharmacokinetic data confirm that 8/40 mg/kg/day divided every 12 hours achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in over 90% of children and matches adult exposure. 1, 4
Formulation Considerations
Use liquid formulation for precise dosing in children under 16 kg when available, though at 21.3 kg this child can reliably use tablets. 1 The liquid suspension contains 40 mg TMP/200 mg SMX per 5 mL, so standard dosing would be approximately 10–11 mL twice daily if liquid is preferred.
Critical Safety Monitoring
Obtain baseline CBC with differential and platelets before starting therapy, then repeat monthly during prolonged treatment to detect hematologic toxicity (neutropenia, thrombocytopenia, anemia). 1, 2
Screen for G6PD deficiency before initiating therapy due to hemolytic anemia risk, particularly in at-risk populations. 1
Ensure adequate hydration (at least 1–1.5 liters daily) to prevent crystalluria, especially with higher doses. 1
Avoid use in children under 2 months due to kernicterus risk. 2, 3
Important Drug Interactions and Precautions
Use caution and consider dose adjustment in patients with:
Renal insufficiency: For CrCl 15–30 mL/min, reduce dose by 50%; for CrCl <15 mL/min, use alternative agent or reduce dose by 50% with close monitoring 1, 3
Concurrent methotrexate: Increased methotrexate toxicity risk 1, 2
Warfarin or anticoagulants: Enhanced anticoagulant effect requiring INR monitoring 1, 2
Oral hypoglycemics: Increased hypoglycemia risk 1
Duration of Therapy
Standard treatment courses are 7–10 days for skin/soft tissue infections and uncomplicated UTIs, with 5 days sufficient for shigellosis. 1, 2, 3 Severe infections like osteomyelitis require >6 weeks of therapy, typically combined with rifampin. 1
Common Pitfalls to Avoid
Underdosing in serious infections: The standard 8 mg/kg/day may be insufficient for resistant pathogens (MIC 1 mg/L), requiring escalation to 12–15 mg/kg/day 1, 4
Failure to adjust for renal impairment: Inadequate dose reduction with CrCl <30 mL/min markedly increases toxicity risk 1
Insufficient monitoring during prolonged therapy: Monthly CBC is essential to detect bone marrow suppression early 1, 2