Phenytoin has a wide volume of distribtuion and is 90% bound to protein.
Only the unbound proportion is biologically active, and therefore, FREE levels of phenytoin should be checked to determine whether acute seizure activity is related to suboptimal levels or not.
Given these properties, patients with the following conditions are more likely to become phenytoin toxic: neonates and the elderly (due to poor metabolism and low protein production); uremia,nephrotic syndrome, pregnancy, malignancy, malnutrition (due to low potein levels).
