Is Precipitation with Sulphosalicylic Acid Valid for Obtaining Free Valproic Acid and Phenytoin

Abstract

The aim of this study was to compare ultrafiltration with protein precipitation with sulphosalicylic acid in the separation of free and protein-bound serum phenytoin and valproic acid. Blood samples from 49 epileptic patients chronically treated with phenytoin (27) and valproic acid (22) were assayed. Free phenytoin or valproic acid were determined by fluorescent polarization immunoanalysis. The results showed that the free serum concentrations obtained by protein precipitation (24.9 ± 8.3 and 2.55 ± 1.61 mg L−1 for valproic acid and diphenylhydantoin, respectively) were almost three-fold those from ultrafiltration (7.5 ± 6.2 and 0.86 ± 0.6 mg L−1, respectively). Protein separation using sulphosalicylic acid is not a valid technique for obtaining the free serum concentration of phenytoin or valproic acid.