Effect of plasma exchange on phenytoin plasma concentration

The effect of plasma exchange on the concentrations of total and unbound phenytoin (DPH) was determined in one normal subject and four patients undergoing therapeutic plasma exchange (PE). The average percentage of plasma exchanged ranged from 57 to 70% among the five patients studied. The reduction in DPH concentration was more variable and ranged from 10 to 16%. However, the extent of reduction within each patient for both total DPH and "free" DPH serum concentrations was minimal; in addition, the ratio of free to total DPH remained virtually unchanged by plasma exchange. Thus, the replacement of endogenous albumin with exogenous albumin, which is an integral part of therapeutic plasma exchange, does not appear to alter DPH binding. Our findings indicate that rapid equilibration of DPH occurs between the intra- and extravascular compartments and that for the occasional plasma exchange procedure, dose adjustment does not appear indicated.