Septra Suspension Dosing Calculation for 20.7kg Child
For a 20.7kg child receiving Septra (trimethoprim-sulfamethoxazole) suspension 200mg/40mg per 5mL at 10mg/kg/day divided BID, administer 5mL twice daily.
Weight-Based Calculation
The FDA-approved pediatric dosing for urinary tract infections and acute otitis media is 40mg/kg sulfamethoxazole and 8mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours 1.
However, your order specifies 10mg/kg/day BID, which appears to reference the trimethoprim component:
- Patient weight: 20.7kg
- Total daily dose (trimethoprim): 10mg/kg/day × 20.7kg = 207mg/day
- Dose per administration (BID): 207mg ÷ 2 = 103.5mg per dose
Volume Calculation
Given the suspension concentration of 200mg sulfamethoxazole/40mg trimethoprim per 5mL:
- Trimethoprim concentration: 40mg per 5mL = 8mg/mL
- Volume needed: 103.5mg ÷ 8mg/mL = 12.9mL per dose
Critical Dosing Discrepancy
The ordered dose of 10mg/kg/day (trimethoprim) is significantly lower than FDA-approved pediatric dosing, which recommends 8mg/kg trimethoprim per dose (16mg/kg/day total) 1. This represents approximately 63% of the standard dose.
Standard FDA Dosing Would Be:
- Trimethoprim: 8mg/kg × 20.7kg = 165.6mg per dose (every 12 hours)
- Volume: 165.6mg ÷ 8mg/mL = 20.7mL per dose (approximately 20mL BID)
- This aligns with the FDA weight-based table recommendation of 2 tablets (equivalent to approximately 20mL suspension) for a 20kg child 1
Clinical Recommendation
Before administering this medication, clarify the order with the prescriber because:
- The ordered dose (10mg/kg/day total) is substantially below standard pediatric dosing (16mg/kg/day) 1
- Subtherapeutic antibiotic dosing fails to achieve adequate tissue concentrations and promotes antimicrobial resistance 2
- For most pediatric infections requiring TMP-SMX, the standard dose is 40mg/kg/day sulfamethoxazole with 8mg/kg/day trimethoprim, divided every 12 hours 1
If the order is confirmed as written: Administer 12.9mL (round to 13mL) twice daily
If standard dosing is intended: Administer 20mL twice daily (providing 8mg/kg trimethoprim per dose)
Common Pitfall to Avoid
Always verify whether the mg/kg dose refers to the trimethoprim or sulfamethoxazole component, as the combination contains both in a 1:5 ratio. The FDA label bases pediatric dosing on both components: 40mg/kg/day sulfamethoxazole AND 8mg/kg/day trimethoprim 1. An order stating only "10mg/kg/day" creates ambiguity that requires clarification before administration.