How do you determine the maintenance dose for a target Css,avg with a known dosing interval?

• A. MD = Css,avg × Cl × τ.
• B. MD = Css,avg × Vd.
• C. MD = Css,avg / (Cl × τ).
• D. MD = Dose per interval.

Answer: (A)

To maintain a target average concentration (Css,avg) with a fixed dosing interval, you need a dose that replaces what is cleared during each interval. The amount eliminated at Css,avg is Css,avg × Cl, and over one dosing interval τ the total eliminated is Css,avg × Cl × τ. Therefore, the maintenance dose must equal that amount: MD = Css,avg × Cl × τ. This ensures the average level stays steady across intervals, with correct units (mg = mg/L × L/h × h).

Using the volume of distribution would address distribution, not how fast the drug is cleared, so it doesn’t set the maintenance dose. Dividing by Cl × τ would invert the correct relationship and give the wrong magnitude. Simply saying “dose per interval” lacks the necessary link to clearance and the dosing interval needed to achieve the target Css.