An alternative method for calculating absorption and elimination rate constants in first-order processes: application to valproic acid

A simple method for calculating absorption and elimination rate constants from a single linear plot of plasma drug concentration data is presented. This method is applicable to the one-compartment open-body model with first-order processes. It allows an independent determination of the two rate constant values. As the two rate constants are found to be two square-roots of a quadratic equation, the proposed method obviates the fact that it cannot determine the identity of each rate constant from a single extravascular administration of a drug. Supplementary information is needed to complete this identity in order to make the proposed method globally identifiable. An application of the proposed method is presented using plasma data of valproic acid obtained after a single oral administration of the drug to two epileptic patients.