Comparison of four valproic acid free fraction determination methods used in clinical practice.

Objective: Given that hypoalbuminemia tends to result in higher free fraction  concentrations of valproic acid, different methods have been developed to  determine the latter in patients with this condition. The aim of this study is to  assess the reliability of these methods and, if necessary, design a new  estimation method.

Method: A retrospective analysis was carried out by the Pharmacy Department of Severo Ochoa University Hospital of admitted patients with at least one  trough concentration of valproic acid between October 2017 and February  2019.