A graphic method for predicting individual phenytoin levels in an office practice.

A simple method is described for predicting a desired steady-state phenytoin plasma level based on two steady-state phenytoin plasma levels achieved at different daily doses. In this graphic method, radial vectors corresponding to specific concentrations are plotted on a clearance-versus-dose graph. These vectors are used to determine a new phenytoin dose that will predict a desired level directly from the graph. Evaluation in an office practice of this method and a computerization of the equation on which it is based demonstrate that it is a practical aid in individualizing phenytoin therapy.