  The theoretical case is considered where the total amount of plasma protein
is conserved, but the drug is eliminated after its single application. After a
single drug application at time t = 0, the total drug concentration is measured
at times ti,...,tk and the total drug concentration at time ti is denoted by
{\phi}i. Our discussion is limited to one protein binding site. The quantity of
plasma protein ({\Lambda}), the association constant (Ka) and the total
concentration of the drug {\phi}i at time ti are considered as independent
variables. Free drug concentration, plasma protein bound concentration and free
drug fraction are given as functions of these "new" variables. The aim of this
communication is to derive the formula that allows to calculate the free drug
concentration at any time after the drug application, based on 3 parameters:
the association constant of the drug, the total plasma concentration of the
drug and the concentration of the protein. If the plasma protein quantity
({\Lambda}) and the association constant (Ka) are known, then from the
knowledge of the total drug concentration {\phi}i at time ti it is possible to
determine the free drug concentration at time ti.
