What is the recommended dose of Bactrim (trimethoprim/sulfamethoxazole) for a pediatric female patient weighing 45lbs (20.4 kg)?

Bactrim Dosing for 45 lb Pediatric Female

For a 45 lb (20.4 kg) pediatric female, the recommended Bactrim dose is 20 mL of the 40/200 mg/5mL suspension (or 400 mg sulfamethoxazole/80 mg trimethoprim) twice daily for standard treatment indications. 1, 2

Weight-Based Dosing Calculation

• The standard pediatric treatment dose is 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim, divided into two doses every 12 hours 2
• For a 20.4 kg child, this calculates to:
  • Sulfamethoxazole: 816 mg/day (408 mg per dose)
  • Trimethoprim: 163 mg/day (81.6 mg per dose)
• This corresponds to one 400/80 mg tablet (or 20 mL suspension) twice daily 1, 2

Indication-Specific Considerations

For Urinary Tract Infections or Acute Otitis Media

• Duration: 10 days 2
• Dose remains 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim divided every 12 hours 2

For Shigellosis/Dysentery

• Duration: 5 days 1, 2
• Same dosing as above 2

For Severe Pneumonia (Pneumocystis jiroveci)

• Treatment dose: 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim divided every 6 hours 2
• For this 20 kg child at upper limit: approximately 2 tablets (400/80 mg) every 6 hours 2
• Duration: 14-21 days 2

Critical Safety Screening

Before prescribing, verify the patient does NOT have:

• G6PD deficiency (risk of hemolytic anemia) 1
• Renal impairment: If creatinine clearance 15-30 mL/min, reduce dose by 50%; if <15 mL/min, avoid use 1, 2
• Recent sulfonamide use (if using topical anesthetics like EMLA) 3
• Age <2 months (contraindicated) 2

Formulation Options

• Oral suspension: 40 mg trimethoprim/200 mg sulfamethoxazole per 5 mL (preferred for accurate dosing in this weight range) 1
• Tablets: 80 mg trimethoprim/400 mg sulfamethoxazole (single strength) 2
• At 45 lbs, the child can receive one single-strength tablet twice daily 2

Common Pitfall to Avoid

The most critical error is failing to document the patient's weight in kilograms on the prescription 4. Without this, the community pharmacist cannot verify appropriate weight-based dosing and must rely on assumptions, compromising patient safety 4. Always include "20.4 kg" or "45 lbs" on the prescription 4.

Monitoring During Treatment

• Baseline and intermittent complete blood counts if prolonged therapy (>14 days) 1
• Monitor for rash, gastrointestinal disturbances, and hematologic abnormalities 5
• If life-threatening toxicity occurs, permanently discontinue 5