Prediction of phenytoin dosage in relation to the variability of phenytoin plasma concentration.

1 In a model study the influence of some variables on the accuracy of predictions of phenytoin doses which should produce desired plasma concentrations were studied. All predictions were extrapolations from low test doses with concurrent observations of plasma concentrations. 2 The variables in introduced were:--Variability in observed plasma concentration;--Variability in applied test doses;--Variability in patient characteristics as expressed in Km and Vmax values. 3 The model study generated the same sort of result as a previous one, executed in practice (Driessen. Van der Velde + Höppener, 1980), i.e. predictions are often inaccurate and of the method tested (Richens & Dunlop, 1975; Ludden, Hawkins, Allen & Hoffman, 1976; Martin, Tozer, Sheiner & Riegelman, 1977; Rambeck, Boenigk, Dunlop, Mullen, Wadsworth & Richens, 1980), Richens' first nomogram (Richens & Dunlop, 1975) is to be preferred.