In pharmacology, clearance () is a pharmacokinetic parameter representing the efficiency of drug elimination. This is the rate of elimination of a substance divided by its concentration. The parameter also indicates the theoretical volume of plasma from which a substance would be completely removed per unit time. Usually, clearance is measured in L/h or mL/min. Excretion, on the other hand, is a measurement of the amount of a substance removed from the body per unit time (e.g., mg/min, μg/min, etc.). While clearance and excretion of a substance are related, they are not the same thing. The concept of clearance was described by Thomas Addis, a graduate of the University of Edinburgh Medical School.
Substances in the body can be cleared by various organs, including the kidneys, liver, lungs, etc. Thus, total body clearance is equal to the sum clearance of the substance by each organ (e.g., renal clearance + hepatic clearance + pulmonary clearance = total body clearance). For many drugs, however, clearance is solely a function of renal excretion. In these cases, clearance is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on how the substance is handled by the nephron. Clearance is a function of 1) glomerular filtration, 2) secretion from the peritubular capillaries to the nephron, and 3) reabsorption from the nephron back to the peritubular capillaries. Clearance is variable in zero-order kinetics because a constant amount of the drug is eliminated per unit time, but it is constant in first-order kinetics, because the amount of drug eliminated per unit time changes with the concentration of drug in the blood.
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