Clinical pharmacy
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Predictions of free (unbound) serum phenytoin concentration by three methods were compared with results obtained by the Abbott TDx Free Phenytoin ultrafiltration and fluorescence-polarization immunoassay technique. Data were obtained for hospitalized adults who had been receiving phenytoin for at least five days and were free of renal or hepatic disease. Total phenytoin concentration was determined, and free phenytoin concentration was measured in ultrafiltrate at 25 degrees C. ⋯ There was a strong correlation between actual and predicted free phenytoin concentrations for each of the methods, but all methods were found to lack precision. All methods also exhibited bias, as demonstrated by overprediction of the free concentration; however, none of the methods exhibited bias when the difference between the in vitro temperature of 25 degrees C and the in vivo temperature of 37 degrees C was considered. Because of their poor precision, the three methods evaluated in this study are not recommended for predicting free phenytoin concentration.