Which parameter describes the target steady-state concentration during a constant-rate infusion?

• A. Css,inf
• B. Cmax
• C. AUC
• D. Ke

Answer: (A)

During a constant-rate infusion, drug enters the body at a steady rate and is cleared at a rate proportional to its concentration. When steady state is achieved, input equals output, so the concentration remains constant. The steady-state target concentration for a continuous infusion is given by Css,inf = R0 / CL, where R0 is the infusion rate and CL is clearance. This value represents the level you aim for with a given infusion rate and clearance, and it is the concentration that persists at steady state.

Cmax reflects the peak concentration after a dose or during the distribution phase, not the steady-state target for a continuous infusion. AUC measures overall drug exposure over time, not the constant concentration at steady state. Ke (the elimination rate constant) governs how fast the drug is removed and how quickly steady state is reached, but it does not set the steady-state concentration itself.