Effect of age, height, weight and sex on serum phenytoin concentration in epileptic patients.

1 Steady-state serum phenytoin concentrations were measured in adult epileptic patients receiving a maintenance dose of phenytoin (300 mg daily). 2 Serum phenytoin concentration showed a positive correlation with age. 3 Serum phenytoin concentration showed a negative correlation with body weight and with height. Multiple correlation analysis indicated that body weight influenced the concentration to a much greater degree than height. 4 When corrected for body weight and height, the serum phenytoin concentrations in women were lower than those in men, although the difference was not statistically significant. 5 Although each of these factors contributes to the interindividual variation in serum phenytoin concentrations, the contribution of each is small. Other factors, such as genetic differences and the effect of saturation kinetics, are much more important in determining steady-state concentrations. Adjusting the dose according to the age, weight and height of a patient would achieve only a marginal improvement in therapy.