Which of the following best defines clearance (CL) in pharmacokinetics?

• A. The volume of plasma cleared of drug per unit time.
• B. The amount of drug eliminated per hour.
• C. The fraction of drug absorbed systemically.
• D. The duration until drug is eliminated.

Answer: (A)

Clearance is the volume of plasma from which the drug is completely removed per unit time. It reflects how efficiently the body eliminates the drug, independent of how much drug is present.

In practice, the rate of elimination equals CL times the plasma concentration (ELIMINATION RATE = CL × C). This gives meaningful units like L/hour or mL/min. For example, if CL is 5 L/hour and the plasma concentration is 2 mg/L, the elimination rate is 10 mg/hour.

This concept differs from the other ideas: the amount eliminated per hour is the elimination rate (not CL itself), the fraction absorbed is bioavailability, and the duration until elimination relates to the half-life and depends on both clearance and volume of distribution, not clearance alone.