Determination of free valproic acid: Evaluation of the centrifree system and comparison between high-performance liquid chromatography and enzyme immunoassay

Valproic Acid (VPA) is an important drug for the treatment of several types of seizures because it has a wide spectrum of activity. Since VPA has an unusual nonlinear binding characteristic and a wide interindividual variation, monitoring of its free concentration can be helpful in patient management. The determination of unbound VPA is more difficult because an extra sample preparation step is needed and the concentration of free VPA is low. Free drug monitoring can assume a more important role if there is a refinement in the technology. A high-performance liquid chromatography (HPLC) method with isocratic elution has been established for the analysis of the 4-bromomethyl-7-methoxycoumarin (BrMMC) derivative of free VPA. This method has a better sensitivity, linearity, and precision than enzyme immunoassay (EIA). Ultrafiltration with the Centrifree system was evaluated for the sample preparation. The influence of centrifuge times, relative centrifugal forces, and the starting sample amounts on the final results of the ultrafiltration were investigated. There was a satisfactory correlation between the free VPA levels determined by the HPLC method and the concentrations obtained by EIA. The total and free VPA were determined on 100 samples from 36 patients. The total VPA levels were in a range of 25 to 208 μg/ml, free VPA concentrations ranged from 1.92 to 55.75 μg/ml with the free fractions from 7 to 37%.