Rebound of plasma free phenytoin concentration following plasmapheresisin a patient with thrombotic thrombocytopenic purpura.

OBJECTIVE

To measure the total and free serum phenytoin concentrations and phenytoin clearance in a patient with thrombotic thrombocytopenic purpura undergoing plasmapheresis.

CASE SUMMARY

A dose of phenytoin 400 mg i.v. was administered daily 2.5 hours after the completion of plasmapheresis. Intravenous samples were obtained to measure the free and total phenytoin concentrations on 4 consecutive days. These samples were obtained before, at the midpoint, upon completion, and 30, 60, and 120 minutes after completion of plasmapheresis. The samples were centrifuged and stored at 4 degrees C until analysis using the same fluorescence polarization immunoassay kit. The free phenytoin was separated by ultracentrifugation.

DISCUSSION

Despite a decrease in total and free serum phenytoin concentrations measured during the procedure, a marked rebound in the free phenytoin concentration was noted 30 minutes after completion of the procedure. However, by 120 minutes after the procedure, the free phenytoin concentration had decreased to the preprocedure value. The total amount of phenytoin removed by the procedure was less than 5%.

CONCLUSIONS

Despite the decrease in total and free serum phenytoin concentrations observed during plasmapheresis, patients undergoing plasmapheresis may not need supplemental phenytoin, as a rapid reequilibration of the therapeutically active free serum phenytoin 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 procedure, a free serum phenytoin concentration may be obtained, as this concentration may reflect the reason for toxicity despite a decreased total phenytoin concentration.