Theorem on the Apparent Volume of Distribution and the Amount of Drug in the Body at Steady State

For an N‐compartmental system, with irreversible drug loss from the sampled compartment, the equilibrium concentration, C(∞), obtained with a zero‐order drug input is related to the total amount of drug in the system, T, by C(∞) V = T. The scalar V is the volume of distribution of the corresponding closed system. The moment functions of the open system define V, and hence T is directly calculable. The derivation is general in the sense that the topology of the system is not specified and no functional form for C(t) is required.