Factors affecting the free plasma fraction of phenytoin in patients with epilepsy.

The variability and possible factors modifying the free plasma fraction of phenytoin were investigated in 134 patients with epilepsy undergoing long-term treatment. The total and free plasma concentrations of phenytoin were determined using fluorescent polarisation immunoassay. Concentrations of albumin, bilirubin and creatinine were also obtained. The free plasma concentration was separated by ultrafiltration, at 25 degrees C, using Centrifree((R)) filters. Factors related to the free plasma fraction of phenytoin (free plasma concentration/total plasma concentration) were gender, age, dose, therapeutic regimen, total plasma concentration and the biochemical parameters mentioned. The mean of free plasma fraction was 9.1% with a very high variability (between 3.3 and 37%). No significant relationship was found between the free plasma fraction and dose, age, gender, total plasma concentration or the biochemical data. The only variable with a significant effect (p < 0.05) on the free plasma fraction was polytherapy with valproic acid. The variability in the free plasma fraction of phenytoin was high in epileptic patients, and was poorly related to the clinical or analytical variables studied. In the absence of pathologies that modify phenytoin binding (uraemia, hypoalbuminaemia), the only factor predictive of a possible alteration in the binding of phenytoin to plasma proteins was polytherapy with valproic acid.