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The clinical value of free phenytoin levels

1985; Wiley; Volume: 18; Issue: 1 Linguagem: Inglês

10.1002/ana.410180116

1531-8249

William H. Theodore, Linda Chia‐Hui Yu, Beth Price, Wayne D. Yonekawa, Roger J. Porter, Izet M. Kapetanović, Henry Moore, Harvey J. Kupferberg,

Pharmaceutical studies and practices

Abstract The relationship between total and free phenytoin levels and drug toxicity was studied in 80 patients. Twenty‐four were taking phenytoin alone. Drug toxicity was assessed by a “blind” rater using an eight‐point standardized scoring system. The mean free phenytoin fraction was 0.076 in patients taking phenytoin alone or phenytoin and carbamazepine and 0.11 in patients taking valproic acid ( p < 0.001). The free fraction did not change with the total level over the range tested (6.7 to 39.9 μg/ml total phenytoin). There was a strong correlation between free and total levels ( r = 0.84). Both free ( r = 0.59) and total ( r = 0.49) phenytoin levels were positively correlated with the toxicity score. Only total phenytoin levels showed a weak positive correlation with decreasing seizure frequency. Our results suggest that routine free phenytoin level monitoring is not necessary in most clinical situations.