If the drug concentration is 5 mg/mL and the prescribed dose is 25 mg per hour, what is the infusion rate in mL per hour?

• A. 5 mL/hr
• B. 1 mL/hr
• C. 10 mL/hr
• D. 0.5 mL/hr

Answer: (A)

Infusion rate in mL per hour is found by dividing the prescribed dose per hour by the drug’s concentration. Use the formula: rate (mL/hr) = dose rate (mg/hr) ÷ concentration (mg/mL). Here, you need 25 mg each hour, and the solution has 5 mg per mL. 25 mg/hr ÷ 5 mg/mL = 5 mL/hr. So delivering 5 mL per hour provides 25 mg per hour. If you went slower or faster, you’d deliver different amounts—for example, 1 mL/hr would be 5 mg/hr, 10 mL/hr would be 50 mg/hr, and 0.5 mL/hr would be 2.5 mg/hr—none of which meet the 25 mg/hr requirement.