Clearance:drug elimination Clearance(Cl)is the most fundamental expression of drug elimination.It is defined as the volume of body fluid (blood)from which a drug is removed per unit of time.Whereas the clearance of a particular drug is constant,it is important to note that the amount of drug contained in the clearance volume will vary with the plasma drug concentration.Clearance: drug elimination Clearance (Cl) is the most fundamental expression of drug elimination. It is defined as the volume of body fluid (blood) from which a drug is removed per unit of time. Whereas the clearance of a particular drug is constant, it is important to note that the amount of drug contained in the clearance volume will vary with the plasma drug concentration