What is the loading dose to achieve a target Css for an IV infusion?

• A. Css × Vd
• B. Rate_in × Vd
• C. Css / Vd
• D. Rate_in / Cl

Answer: (A)

To reach a target Css quickly during an IV infusion, the loading dose should fill the distribution volume to that concentration. At steady state, the amount of drug in the body equals Css × Vd. So the loading dose, which is the amount given upfront, must be Css × Vd to achieve the desired Css immediately. After the loading dose, you maintain Css with a continuous infusion at a rate equal to Css × Cl. The other options don’t give a valid upfront dose: they mix dosage with rate or involve incorrect units, and Rate_in / Cl yields the maintenance rate, not the loading dose.