A child who weighs 18 kg is to receive 900 mg of an antibiotic IV q6h. The antibiotic is available in 1.5 g/vial and it is to be reconstituted to a concentration of 375 mg/mL. How many mL per dose should the child receive?

• A. 0.6
• B. 1.6
• C. 2.4
• D. 6.9

Answer: (C)

This question tests dosing by concentration. To administer a specific milligram dose when the solution concentration is known, convert the dose into milliliters by dividing the required milligrams by the concentration (mg/mL).

The child needs 900 mg per dose, and the reconstituted solution is 375 mg/mL. Volume to give = 900 mg ÷ 375 mg/mL = 2.4 mL.

One vial provides 1500 mg, which at 375 mg/mL yields 1500 ÷ 375 = 4 mL total, so the 900 mg dose fits within a single vial’s reconstituted volume (2.4 mL used, 1.6 mL remaining in the vial).

The weight of the child isn’t needed here since the dose is given in mg per dose rather than per kg.