Valproic acid quantification in human serum by LC-MS/MS: method development and comparison with immunoassay

Valproic acid (VPA) is widely used for epilepsy management, however VPA serum concentration must be monitored because its narrow therapeutic range. Some methods are used to determine the plasma or serum VPA concentration, among them immunoassay-based tests are the most used. Although immunoassays may present cross-responses with other drugs or AE metabolites, increasing the uncertainty of the method. For this reason, a liquid chromatography-mass spectrometry in tandem (LC-MS/MS) method was developed, validated and compared with particle-enhanced turbidimetric inhibition immunoassay (PETINIA). For method optimization, the protein precipitation solvent was methanol and isopropanol (3:1) and a biphenyl column (50 x 3.0 mm, 2.7 μm) was used. The method showed selectivity and specificity, and it was linear from 25 to 450 µg/mL, presenting variation coefficient lower than 8,4% and inaccuracy within 10,9%. The matrix effect was 103.9%. The VPA quantification by both methods had good correlation and the VPA assay by LC-MS/MS was 6,85% higher when compared to the immunoassay, but it still gives the same clinical stratification. The validation was successful, with easy sample preparation and appropriate selectivity, applicable for VPA therapeutic drug monitoring.