To maintain a target Css with IV infusion, what is the maintenance dosing rate?

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

Answer: (B)

Maintaining a target plasma concentration during IV infusion comes down to balancing what goes in with what comes out. At steady state, the rate of drug entering the body equals the rate of drug being eliminated. The rate of elimination is the product of clearance and the target concentration, so the maintenance infusion rate must be Cl × Css. In other words, you inject at a rate that matches how quickly the drug is cleared at the desired concentration.

Volume of distribution, Vd, affects how long it takes to reach Css, not the amount you must infuse per unit time to keep Css steady. The other expressions don’t fit because they either mix units incorrectly or represent total amount rather than a rate.