Pediatric Bactrim Liquid Dosing Calculation
For a 69-pound (31.3 kg) child using 10 mg/kg trimethoprim dosing with a 40 mg/5 mL suspension, administer 7.8 mL (approximately 8 mL) twice daily.
Weight Conversion and Dose Calculation
The child weighs 69 pounds, which equals 31.3 kg (69 ÷ 2.2 = 31.3 kg). 1
At 10 mg/kg trimethoprim dosing: 31.3 kg × 10 mg/kg = 313 mg trimethoprim per day. 1
This daily dose should be divided into two doses (every 12 hours), giving 156.5 mg trimethoprim per dose. 2, 1
Volume Calculation with Available Concentration
The suspension concentration is 40 mg trimethoprim per 5 mL. 2
To calculate the volume per dose: (156.5 mg ÷ 40 mg) × 5 mL = 19.6 mL per day or 9.8 mL per dose when divided twice daily. However, using the standard 8 mg/kg dosing (which is more commonly recommended for mild-to-moderate infections): 1
At 8 mg/kg: 31.3 kg × 8 mg/kg = 250.4 mg trimethoprim per day, divided into two doses = 125.2 mg per dose. 1
Volume per dose: (125.2 mg ÷ 40 mg) × 5 mL = 15.7 mL per dose (approximately 16 mL twice daily). 1
Clinical Context for Dosing Selection
For mild-to-moderate infections (urinary tract infections, uncomplicated skin infections, otitis media):
- Use 8–10 mg/kg/day trimethoprim divided every 12 hours, which translates to 10–12 mL of suspension every 12 hours for this weight. 1
For serious infections (severe MRSA, complicated infections):
- Use 10–12 mg/kg/day trimethoprim divided every 12 hours, which translates to 14–16 mL of suspension every 12 hours. 1
For life-threatening infections (severe pneumonia, CNS infections):
- Consider 15–20 mg/kg/day trimethoprim divided every 6–8 hours (four times daily dosing). 1
Practical Dosing Recommendation
For standard treatment at 10 mg/kg/day: Administer approximately 8 mL twice daily (rounding 7.8 mL for practical measurement). 1
For standard treatment at 8 mg/kg/day: Administer approximately 7.8 mL twice daily (can round to 8 mL). 1
The liquid formulation is appropriate for accurate dosing in pediatric patients, particularly those under 40 kg. 1
Important Monitoring and Safety Considerations
Obtain baseline complete blood count with differential and platelet count at treatment initiation, and repeat monthly during prolonged therapy to monitor for hematologic toxicity (neutropenia, thrombocytopenia, anemia). 1
Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria, especially during high-dose therapy. 1
Screen for G6PD deficiency before initiating therapy due to hemolytic anemia risk. 1
Common adverse effects include rash, gastrointestinal disturbances, and hematologic abnormalities; if life-threatening reactions occur (Stevens-Johnson syndrome, anaphylaxis), permanently discontinue the medication. 3, 1
Standard treatment duration for most bacterial infections is 7–14 days. 1