Plasma drug concentrations: description and interpretation of the biexponential decay.

Information about the time course of the plasma concentrations of a drug after its i.v. bolus injection is necessary for understanding the time course of the effect of the drug. If an investigator has established that the time course of the plasma concentrations conforms to a biexponential function, understanding the data is facilitated by description of the time course which simply summarizes the data in a quantitative fashion and interpretation of the findings based on certain assumptions. Interpretation may be based on the assumption of compartments, or on a "whole body" approach. The experimentally obtained plasma concentrations of a drug may be described quantitatively by an equation and, if the "whole body" interpretation is selected in preference tot he compartmental interpretation, the four parameters are: the initial and the terminal volumes of distribution (Vc and Vterm) and the rate constants for the loss from Vc and for the dilution within the whole body (k(loss) and k(mix), respectively).