Which equation defines Css for an IV infusion?

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

Answer: (B)

For a constant IV infusion, the steady-state concentration is determined by the balance between how much drug is entering per unit time and how much is being cleared per unit time. The steady-state concentration, Css, equals the infusion rate divided by clearance: Css = Rate_in / Cl. This works because Rate_in has units of mass per time and Cl has units of volume per time, so their ratio gives mass per volume (concentration).

If you change the infusion rate, Css changes in proportion to that change; if clearance increases, Css decreases because more drug is eliminated per hour.

The distribution volume, Vd, does not set Css at steady state. It influences how long it takes to reach Css (the time to steady state is related to Vd/Cl), but not the actual concentration once steady state is achieved. The other ratios do not produce a concentration at steady state: multiplying by Cl would not yield concentration, dividing by Vd ignores elimination, and Vd/Cl gives units of time rather than concentration.