The clinical value of free phenytoin levels

✍ Scribed by Dr. William H. Theodore; Dr. L. Yu; Beth Price; Wayne Yonekawa; R. J. Porter; I. Kapetanovic; Henry Moore; Harvey Kupferberg

• Publisher: John Wiley and Sons
• Year: 1985
• Tongue: English
• Weight: 362 KB
• Volume: 18
• Category: Article
• ISSN: 0364-5134
• DOI: 10.1002/ana.410180116

✦ Synopsis

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. T h e mean free phenytoin fraction was 0.076 in patients taking phenytoin alone or phenytoin and carbamazepine and 0.1 1 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 kg/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.