How do you calculate a prescribed dose when given desired dose and stock concentration? Include a sample calculation.

• A. Dose desired ÷ stock dose × quantity on hand; Example: to give 2 mg from a 5 mg/mL ampule, administer 0.4 mL.
• B. Stock dose ÷ dose desired × quantity on hand; Example: to give 2 mg from a 5 mg/mL ampule, administer 0.4 mL.
• C. Dose desired × stock dose ÷ quantity on hand; Example: to give 2 mg from a 5 mg/mL ampule, administer 4 mL.
• D. Dose desired ÷ stock dose × 2; Example: 2 mg from a 5 mg/mL ampule, administer 0.8 mL.

Answer: (A)

To figure out how much to give when you have a desired dose and a stock concentration, focus on how many milliliters contain the amount you need. The volume to administer is the desired dose (in mg) divided by the stock concentration (in mg per mL). In other words, volume (mL) = desired dose (mg) ÷ stock concentration (mg/mL). This works because the mg units cancel, leaving mL as the result. If you’re pulling from a vial with a certain amount on hand, you can multiply by the quantity on hand to determine the total volume you should withdraw from the stock.

Sample calculation: you want 2 mg, and the stock concentration is 5 mg/mL. Volume to give = 2 mg ÷ 5 mg/mL = 0.4 mL. If you have one ampule on hand, you’d administer 0.4 mL from it. If you have two ampules on hand and need 0.8 mL total, you’d withdraw 0.4 mL from each appropriately.

The other approaches mix up the order or multiply by the wrong factor, leading to an incorrect volume (for example, dividing the stock dose by the desired dose or multiplying by an extra factor would not yield 0.4 mL in the given scenario).