Ampicillin 250 mg IV every 6 hours via piggyback. Stock is 1000 mg/50 mL. How many mL per dose?

• A. 5 mL
• B. 10 mL
• C. 12.5 mL
• D. 15 mL

Answer: (C)

This question checks how to turn a milligram dose into the volume to administer using a given stock concentration. Start by finding how many milligrams are in each milliliter of the stock: 1000 mg in 50 mL equals 1000/50 = 20 mg per mL. To deliver 250 mg, the required volume is 250 mg ÷ 20 mg/mL = 12.5 mL. So the dose volume is 12.5 mL. For reference, 5 mL would only provide 100 mg, 10 mL would be 200 mg, and 15 mL would be 300 mg, so 12.5 mL uniquely yields the correct 250 mg.