Bactrim Dosing for 21.6 kg Child
For a child weighing 21.6 kg, administer 5.4 mL of liquid Bactrim (200mg/40mg per 5mL) twice daily, which provides approximately 8-9 mg/kg/day of trimethoprim.
Dosing Calculation
The FDA-approved pediatric dosing for most common infections (UTI, otitis media, shigellosis) is 8 mg/kg/day of trimethoprim component divided into two doses every 12 hours 1. The IDSA guidelines for skin and soft tissue infections recommend 8-12 mg/kg/day of trimethoprim in 2-4 divided doses 2.
Step-by-Step Calculation:
- Target dose: 8 mg/kg/day trimethoprim
- Child's weight: 21.6 kg
- Total daily trimethoprim needed: 8 mg/kg × 21.6 kg = 172.8 mg/day
- Dose per administration (BID): 172.8 mg ÷ 2 = 86.4 mg trimethoprim
Since your liquid formulation contains 40 mg trimethoprim per 5 mL:
- Volume needed per dose: (86.4 mg ÷ 40 mg) × 5 mL = 10.8 mL twice daily
However, if your formulation is labeled as "200/40mg" meaning 200mg sulfamethoxazole and 40mg trimethoprim per 5mL (the standard suspension concentration):
- Volume needed per dose: (86.4 mg ÷ 40 mg) × 5 mL = approximately 10.8 mL twice daily
Alternative Dosing Based on Indication
For Severe SSTI or MRSA Coverage:
The IDSA guidelines support 8-12 mg/kg/day trimethoprim 2, 3. For more serious infections, you may use the higher end:
- 12 mg/kg/day: 259.2 mg/day total = 129.6 mg per dose = 16.2 mL twice daily
For PCP Treatment (if applicable):
The FDA label specifies 15-20 mg/kg trimethoprim per 24 hours in divided doses every 6 hours 1, which would require substantially higher doses and is not typically given as twice-daily dosing.
Important Caveats
- Contraindication: Bactrim is contraindicated in children under 2 months of age 1
- Renal function: Ensure normal renal function; dose reduction required if creatinine clearance <30 mL/min 1
- Duration: Typical treatment courses are 5-14 days depending on indication 1
- Monitoring: Watch for rash, GI upset, and hematologic abnormalities
Practical Dosing Recommendation
The most practical approach for standard infections in this 21.6 kg child is 10 mL twice daily (rounding 10.8 mL for ease of administration), providing approximately 8 mg/kg/day trimethoprim, which aligns with FDA labeling 1 and IDSA guidelines 2.