Which statement best defines clearance and its common units?

• A. Clearance is the amount of drug eliminated per hour.
• B. Clearance is the fraction of drug absorbed.
• C. Clearance is the concentration of drug in plasma.
• D. Clearance is the volume of plasma cleared of drug per unit time; units L/h (or mL/min).

Answer: (D)

Clearance is the body's efficiency in removing a drug from plasma, defined as the volume of plasma from which the drug is completely removed per unit time. This makes clearance a rate expressed as volume per time, with typical units L/h or mL/min. The actual drug elimination rate equals clearance times the current plasma concentration (Elimination rate = CL × C). Because of this relationship, clearance reflects how fast the body can clear a drug independently of the dose, and within linear kinetics changing the dose alters the concentration but not the clearance. This also ties into how half-life depends on both volume of distribution and clearance: t1/2 = 0.693 × Vd / CL. The other ideas mix up different concepts: the amount eliminated per hour is a rate of elimination, not clearance; the fraction absorbed relates to bioavailability; and the concentration in plasma is not clearance.