Estimation of the elimination half-life of a drug at any serum concentration when the Km and Vmax of the drug are known: calculations and validation with phenytoin.

A method of estimating the elimination half-life of a drug at any selected serum concentration when the Km and Vmax of that drug are known is described. The method was validated in six patients by determining their Km and Vmax values for phenytoin, using data obtained at two serum concentrations and then using the Km and Vmax values to predict the elimination half-life of phenytoin at a third serum concentration. The predicted elimination half-lives at the third serum concentration did not differ significantly (t = 1.16, P = .30) from observed elimination half-lives determined with stable isotope tracer techniques.