Clearance and its significance

Clearance (CL) is defined as the volume of blood in a specified region of the body that is cleared of a drug in unit time. It is the parameter that relates the rate of 

The clearance of a drug from a specific region of the body is the sum of all the clearances of all the contributing processes in that region. For example, hepatic clearance (C1H) is the sum of the clearances due to metabolism (C1M) and excretion (ClBiie) in the liver, that is  

Flowever, it is emphasized that clearance is an artificial concept as it is not possible for a drug to be removed from only one part of the total volume of the blood in the body or organ. 

For elimination processes exhibiting first order kinetics it can be shown that clearance is related to Vd, /cd and t1/2 by the mathematical expressions  

Since both l]/2 and /cei are constant for elimination processes following first order kinetics, Cl will also be constant. Flowever, should the order of the elimination change due to a change in the biological situation, such as the drug concentration increasing to the point where it saturates the metabolic elimination pathways, then clearance may not be constant. 

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