Calculating Dexamethasone Volume for 15 mg Dose
To administer 15 mg of dexamethasone from a vial containing 8 mg/2 mL, you need to draw up 3.75 mL.
Step-by-Step Calculation
The calculation follows a straightforward proportion method:
- Given concentration: 8 mg per 2 mL
- Desired dose: 15 mg
- Formula: (Desired dose ÷ Available dose) × Volume = Required volume 1
Calculation:
- (15 mg ÷ 8 mg) × 2 mL = 3.75 mL
Verification of Calculation
To verify this is correct:
- 3.75 mL × (8 mg ÷ 2 mL) = 3.75 mL × 4 mg/mL = 15 mg ✓
Clinical Context for Dexamethasone Dosing
While 15 mg is not a standard dose in most guidelines, understanding common dexamethasone dosing helps contextualize this calculation:
- For PONV prophylaxis: 4-8 mg is the typical range, with 8 mg showing equivalent efficacy to higher doses in major gastrointestinal surgery 2
- For cerebral edema: Initial doses of 10 mg IV followed by 4 mg every 6 hours are standard 1
- For multiple myeloma regimens: 20-40 mg doses are used on specific days 2
- For high-dose therapy: Doses ranging from 40-300 mg have been studied in specific conditions 3, 4
Administration Considerations
Route equivalence: Dexamethasone has 1:1 bioequivalence between IV and oral routes, meaning 15 mg IV = 15 mg PO 5. However, high-dose oral dexamethasone (100-300 mg) shows only 63.4% bioavailability, so this equivalence applies primarily to standard doses 4.
Preparation: The FDA label specifies that dexamethasone sodium phosphate injection can be given directly from the vial or added to IV solutions 1. When drawing up 3.75 mL, use a syringe with 0.1 mL graduations for accuracy.
Timing: For IV administration, the drug can be given as a direct injection or added to an infusion solution 1.
Common Pitfalls to Avoid
- Do not confuse concentration units: The vial states 8 mg/2 mL, which equals 4 mg/mL—always convert to a single unit for clarity
- Do not round prematurely: Calculate the exact volume (3.75 mL) rather than rounding to 3.5 or 4 mL, as this would result in underdosing or overdosing
- Verify the vial concentration: Always double-check the label, as dexamethasone comes in multiple concentrations (commonly 4 mg/mL, 10 mg/mL, or 20 mg/mL) 1