REBOUND OF PLASMA-FREE PHENYTOIN CONCENTRATION FOLLOWING PLASMAPHERESIS IN A PATIENT WITH THROMBOTIC THROMBOCYTOPENIC PURPURA

OBJECTIVE: To measure the total and free serum phenytoin concentration s and phenytoin clearance in a patient with thrombotic thrombocytopeni c purpura undergoing plasmapheresis. CASE SUMMARY: A dose of phenytoin 400 mg iv was administered daily 2.5 hours after the completion of pl asmapheresis. intravenous samples were obtained to measure the free an d total phenytoin concentrations on 4 consecutive days. These samples were obtained before, at the midpoint, upon completion, and 30, 60, an d 120 minutes after completion of plasmapheresis. The samples were cen trifuged and stored at 4 degrees C until analysis using the same fluor escence polarization immunoassay kit. The free phenytoin was separated by ultracentrifugation. DISCUSSION: Despite a decrease in total and f ree serum phenytoin concentrations measured during the procedure, a ma rked rebound in the free phenytoin concentration was noted 30 minutes after completion of the procedure. However, by 120 minutes after the p rocedure, the free phenytoin concentration had decreased to thr prepro cedure value. The total amount of phenytoin removed by the procedure w as less than 5%. CONCLUSIONS: Despite the decrease in total and foe se rum phenytoin concentrations observed during plasmapheresis. patients undergoing plasmapheresis may not need supplemental phenytoin, as a ra pid reequilibration of the therapeutically active fret: serum phenytoi n occurs following the completion of the procedure. In addition, less than 5% of the total body phenytoin is removed. Second if the patient experiences symptoms of phenytoin toxicity immediately after the proce dure, a free serum phenytoin concentration may be obtained, as this co ncentration may reflect the reason for toxicity despite a decreased to tal phenytoin concentration.