Ondansetron Dosing Clarification for an 18kg, 3-Year-Old Child
No, 3.5 mL is not the correct dose—you should give 2.7 mL of ondansetron oral solution (4 mg/5 mL concentration) to deliver the appropriate 2.7 mg dose for this 18 kg child. 1, 2
Weight-Based Calculation
- The standard pediatric dose of ondansetron is 0.15 mg/kg per dose with a maximum of 16 mg 1, 2, 3
- For an 18 kg child: 18 kg × 0.15 mg/kg = 2.7 mg per dose 1, 2
- This dose can be rounded to 4 mg if using ondansetron orally disintegrating tablets (ODT), as the 4 mg tablet is the closest available strength 2
Volume Calculation for Oral Solution
- If using ondansetron oral solution at 4 mg/5 mL concentration (the standard pediatric formulation):
- 2.7 mg ÷ 4 mg × 5 mL = 3.375 mL (which rounds to 3.4 mL, not 3.5 mL) 1
- If using a different concentration (such as 6 mg/mL), the volume would be dramatically different:
- 2.7 mg ÷ 6 mg/mL = 0.45 mL 1
Critical Dosing Considerations
- Always verify the concentration of your ondansetron solution before calculating the volume—this is the most common source of dosing errors 1
- The dose of 0.15 mg/kg provides an appropriate safety margin for QT interval prolongation, which occurs in a dose-dependent manner 1
- This child is well above the 6-month minimum age requirement for ondansetron use 1, 2
Practical Dosing Options
- Option 1 (Preferred): Give one 4 mg ODT tablet, which is easier to administer and provides consistent dosing 2
- Option 2: Give 3.4 mL of 4 mg/5 mL oral solution if ODT is not available 1
- The dose can be repeated every 8 hours as needed 2
Common Pitfall to Avoid
- Do not confuse the concentration of ondansetron oral solution—the 4 mg/5 mL concentration is most common in pediatrics, but 6 mg/mL concentration exists and would require only 0.45 mL for this child 1
- Always double-check your calculation: dose (mg) = weight (kg) × 0.15 mg/kg, then convert to volume based on your specific concentration 1, 2