A prescriber orders a concentration of 0.5 mg/mL and the patient needs a total dose of 4 mg. How many milliliters are required?

A prescriber orders a concentration of 0.5 mg/mL and the patient needs a total dose of 4 mg. How many milliliters are required?

• A. 4 mL
• B. 6 mL
• C. 7 mL
• D. 8 mL

Answer: (D)

Converting a prescribed dose to a measurable volume uses the relationship between dose and concentration. The volume needed equals the dose divided by the concentration. Here the dose is 4 mg and the concentration is 0.5 mg per mL. 4 mg ÷ 0.5 mg/mL = 8 mL. The mg cancels, leaving milliliters. A quick check: if 0.5 mg is in each mL, then 2 mL contain 1 mg; thus 8 mL contain 4 mg. The required volume to administer is 8 mL.

Converting a prescribed dose to a measurable volume uses the relationship between dose and concentration. The volume needed equals the dose divided by the concentration.

Here the dose is 4 mg and the concentration is 0.5 mg per mL. 4 mg ÷ 0.5 mg/mL = 8 mL. The mg cancels, leaving milliliters.

A quick check: if 0.5 mg is in each mL, then 2 mL contain 1 mg; thus 8 mL contain 4 mg. The required volume to administer is 8 mL.