What is Clearance? - Definition
Clearance (Cl) is the volume of blood from which a drug is removed in a given time. Since drug molecules can only be eliminated from plasma it is conventional to refer to plasma clearance. It is perhaps easiest to consider this from the perspective of an eliminating organ, such as the liver. Blood flows through the liver at a rate of 1350 ml/min, giving a hepatic plasma flow of 800 ml/min. If a drug has a hepatic extraction ratio of 0.2, the volume of plasma from which the drug is removed per minute by the liver (the drug's hepatic clearance) is 160 ml/min.
A similar concept applies to the removal of drug by the kidney, and at other sites of elimination, and total plasma clearance is the sum of all these clearance values:
Total clearance can be determined after an intravenous dose by dividing the dose by the area under the plasma concentration-time curve (AUC) (Fig 7). Clearance is the best index of the overall activity of the various elimination processes to which a drug is subject. Clearance describes the volume of plasma cleared of drug per unit of time and so it is independent of the amount of drug in the body and the plasma concentration when elimination is first-order. Clearance (Cl) can be considered as the constant factor that relates the rate of drug elimination to the plasma concentration of the drug (C), as follows:
