The use of marker drugs to measure organ function: a theoretical interpretation.

An equation is presented which describes the changes in drug clearance through an isolated organ due to alterations in the blood flow to the organ or the intrinsic clearance of the drug by the organ. In the case of hepatic clearance, example calculations of alterations in both parameters are presented from literature data for marker drugs having high and low extraction ratios. The predicted hepatic clearance of d-propranolol based on these calculations compares with observed values. For the particular example of the kidney, the blood flow and the intrinsic clearance change in proportion and, therefore, yield a simple equation relating renal clearance of any drug to endogenous creatinine clearance. This equation, though used clinically, has previously not been derived theoretically.