Phenytoin removal by plasmapheresis in renal insufficiency

The removal of phenytoin by plasmapheresis in a patient with compromised renal function is described. The results indicate that plasma concentrations of total phenytoin increase after plasmapheresis, while unbound phenytoin plasma concentrations and free fraction decrease. The amount of phenytoin removed was 36 mg and, when expressed as a fraction of total body stores, equaled 2.8%. The negligible amount of total body phenytoin removed during plasmapheresis was the result of an increased volume of distribution due to renal failure and hypoalbuminemia. Our results indicate that the removal of phenytoin by plasmapheresis is likely to be greatest when its free fraction is the lowest and blood flow the highest. The data suggest that replacement of phenytoin after plasma exchange may be unnecessary but in all cases should be based on unbound concentrations or the determination of the absolute amount of phenytoin removed during the procedure.