What is the equation for the clearance (CL) of a drug?

The correct equation for the clearance (CL) of a drug is derived from its definition in pharmacokinetics. Clearance is the volume of plasma from which a drug is completely removed over a certain period of time. This is mathematically expressed as the rate of elimination of the drug divided by the plasma drug concentration.

This relationship highlights that clearance is fundamentally a measure of the efficiency of the body to eliminate the drug from the system. When the rate of elimination (often expressed in units such as mg/hour or L/hour) is divided by the concentration of the drug in the plasma (measured in mg/L or similar units), it gives a result that indicates the volume of plasma from which the drug can be cleared per unit time. Thus, using this formulation allows clinicians and pharmacologists to assess how well a patient is metabolizing and excreting the medication.

This equation is crucial in various clinical scenarios, such as adjusting drug dosages in patients with altered clearance due to kidney or liver dysfunction, and it is foundational in understanding drug kinetics in the human body.