Bactrim Dosing for a 70 lb Pediatric Patient
For a 70 lb (32 kg) pediatric patient with normal renal function, administer Bactrim at 8-12 mg/kg/day of trimethoprim (256-384 mg TMP/day) and 40-60 mg/kg/day of sulfamethoxazole (1,280-1,920 mg SMZ/day), divided into two doses given every 12 hours. 1, 2, 3, 4
Practical Dosing Translation
For this 70 lb child, the standard dose is 2 single-strength tablets (or 1 double-strength tablet) every 12 hours for most infections. 4
The FDA-approved dosing table specifically indicates that for children weighing 88 lb (40 kg), the dose is 2 single-strength tablets or 1 double-strength tablet every 12 hours; a 70 lb (32 kg) child falls just below this weight bracket, making this the appropriate starting dose. 4
Indication-Specific Adjustments
For Mild-to-Moderate Infections
- Use 8-10 mg/kg/day of trimethoprim component (approximately 256-320 mg TMP/day for this patient), divided every 12 hours for urinary tract infections, skin and soft tissue infections, or acute otitis media. 3
- Duration: 10-14 days for UTIs, 7-10 days for skin infections. 2, 4
For Severe Infections (MRSA)
- Use 10-12 mg/kg/day of trimethoprim component (320-384 mg TMP/day), or up to 15-20 mg/kg/day divided every 6-8 hours for life-threatening infections. 3
- For severe MRSA osteomyelitis specifically, consider 4 mg/kg/dose every 8-12 hours, typically combined with rifampin for >6 weeks. 3
For Pneumocystis Jiroveci Pneumonia (PCP)
- Treatment: 75-100 mg/kg/day sulfamethoxazole (2,400-3,200 mg SMZ/day) and 15-20 mg/kg/day trimethoprim (480-640 mg TMP/day), divided every 6 hours for 14-21 days. 4
- Prophylaxis: 150 mg/m² TMP and 750 mg/m² SMZ daily, divided twice daily, given 3 consecutive days per week. 1
Formulation Considerations
Use liquid formulation for more accurate dosing in younger children, particularly those weighing <16 kg (35 lb). 3 However, for this 70 lb patient, tablets are appropriate and easier to administer.
Each single-strength tablet contains 80 mg trimethoprim and 400 mg sulfamethoxazole; each double-strength tablet contains 160 mg trimethoprim and 800 mg sulfamethoxazole. 4
Pharmacokinetic Evidence Supporting This Dosing
Research demonstrates that 8-12 mg/kg/day divided every 12 hours achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children and matches adult exposure. 5 This dosing strategy is supported by population pharmacokinetic modeling showing that weight-based dosing in children produces comparable drug exposure to standard adult dosing. 5
Critical Safety Monitoring
- Obtain baseline complete blood count with differential and platelet count before starting therapy. 2, 3
- Repeat CBC monthly during prolonged therapy to monitor for hematologic toxicity including neutropenia, thrombocytopenia, and anemia. 2, 3
- Monitor for dermatologic reactions, particularly rash; discontinue permanently for urticarial rash or Stevens-Johnson syndrome. 2
Important Contraindications and Precautions
- Contraindicated in children <2 months of age due to kernicterus risk. 2, 4
- Use with extreme caution in G6PD deficiency due to hemolytic anemia risk. 1, 3
- Avoid concurrent use with methotrexate at treatment doses due to severe bone marrow suppression risk. 1, 3
- May enhance anticoagulant effect of warfarin and increase hypoglycemia risk with oral hypoglycemics. 2, 3
Renal Adjustment
For this patient with normal renal function, no adjustment is needed. However, if creatinine clearance falls below 30 mL/min, reduce to half the usual dose; avoid use if CrCl <15 mL/min. 4