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The clinical value of free phenytoin levels
Author(s) -
Theodore William H.,
Yu L.,
Price Beth,
Yonekawa Wayne,
Porter R. J.,
Kapetanovic I.,
Moore Henry,
Kupferberg Harvey
Publication year - 1985
Publication title -
annals of neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.764
H-Index - 296
eISSN - 1531-8249
pISSN - 0364-5134
DOI - 10.1002/ana.410180116
Subject(s) - phenytoin , carbamazepine , toxicity , free fraction , medicine , anticonvulsant , valproic acid , anesthesia , pharmacology , epilepsy , pharmacokinetics , psychiatry
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.