A rapid new ultrafiltration technique (EMIT FreeLevel System I) for the routine measurement of unbound phenytoin concentrations was evaluated. The precision of this procedure was sufficient (between-days coefficient of variation, 11.7%). The results obtained by the ultrafiltration technique for the percentage of free phenytoin in samples from about 40 non-uraemic patients treated with this drug were in good agreement with those determined by ultracentrifugation and equilibrium dialysis (ultrafiltration vs ultracentrifugation, y = 0.94x + 0.60%; ultrafiltration vs equilibrium dialysis, y = 1.02x - 0.60%). The mean value of the results obtained by ultracentrifugation was significantly about 8% lower than that observed with equilibrium dialysis, apparently due to a sedimentation of free phenytoin during ultracentrifugation. With the methods used in our study, the mean values of percentage phenytoin bound to serum proteins obtained in samples from non-uraemic patients ranged from 91.8 to 92.8%. All 3 methods yielded similar binding curves for phenytoin, with spiked human pool sera containing albumin concentrations between 19 and 45 g/L. A rise of the unbound phenytoin fraction was observed with increasing total concentrations of the drug and a decrease of the albumin concentration. With samples from non-uraemic patients (n = 203), a rather good correlation was found between free and total phenytoin concentrations (r = 0.91). In a number of patients (n = 11), free phenytoin concentrations correlated better than total phenytoin concentrations with the clinical status. Patients with free phenytoin concentrations of less than or equal to 2.1 micrograms/ml and total phenytoin concentrations above the therapeutic range did not show signs of toxicity. From the results of our study it is concluded that the EMIT FreeLevel ultrafiltration technique is very well-suited for a rapid and reliable separation of unbound phenytoin. In patients with altered protein binding or an unusual clinical response, free phenytoin determinations appear to be necessary for proper interpretation of total phenytoin levels and rational dosage adjustment.
Adolescent Adult