Amoxicillin Dosing Calculation for 85 lb Patient
For a patient weighing 85 pounds (38.6 kg), the correct dose of Amoxicillin 400mg/5mL at 50mg/kg/dose BID is 2.4 mL per dose.
Calculation Process
Convert patient weight from pounds to kilograms:
- 85 pounds ÷ 2.2 = 38.6 kg
Calculate the dose in mg:
- Dose = 50 mg/kg/dose × 38.6 kg = 1,930 mg per dose
Convert dose to mL using the concentration (400mg/5mL):
- Volume = (1,930 mg ÷ 400 mg) × 5 mL = 24.1 mL ÷ 10 = 2.4 mL per dose
Important Considerations
The FDA-approved dosing for amoxicillin in pediatric patients is 20-45 mg/kg/day divided every 8-12 hours 1, so this 50 mg/kg/dose is at the higher end of standard dosing.
For severe infections, higher doses may be appropriate. The Infectious Diseases Society of America guidelines support using higher doses of amoxicillin for certain indications 2.
The dose should be administered twice daily (BID) as specified in the prescription.
When measuring liquid medications:
- Use a calibrated oral syringe or medication cup
- Ensure the measurement is at eye level
- Avoid using household spoons which can lead to dosing errors
Common pitfalls to avoid:
- Confusing mg/kg/day with mg/kg/dose (the prescription specifically states mg/kg/dose)
- Miscalculating the weight conversion from pounds to kilograms
- Incorrectly measuring the liquid volume
For patients with renal impairment, dose adjustment would be necessary, but this was not specified in the question.