Determination of Valproic Acid in Plasma or Serum by Solid-Phase Column Extraction and Gas-Liquid Chromatography

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Abstract
Summary: A simple, fast, sensitive, and inexpensive method is described for routine therapeutic monitoring of both total and free valproic acid (VPA) at therapeutic concentrations in seizure patients. The method uses solid-phase (Bond Elut) columns for the preliminary extraction of 100 μl plasma or serum and gasliquid chromatography (GLC) analysis, using 10% Carbowax 6000 on a Chromosorb WAW 80/100 column and α-methyl-α-ethylcaproic acid as an internal standard. The extraction efficiency for VPA at a concentration of 70 μg/ml is 98 ± 4%. The calibration curve is linear in the range of 1.0-280 μg/ml. The detection limit of VPA is 1.0 μg/ml. Syva enzyme multiplied immunoassay technique (EMIT) Free-Level filters were used in the analysis of free VPA from plasma. The described Bond Elut GLC (BE • GLC) method was used to determine total drug concentrations in 56 VPA-treated patients, and the results were compared with those obtained by conventional liquid-liquid extraction • GLC analysis. The 44 sera involved were also analyzed by EMIT. In addition, BE • GLC was compared with the Abbott TDx system on an additional 21 specimens. Statistical agreement was excellent between BE • GLC and TDx, but poor between BE • GLC and liquid-liquid extraction • GLC. Agreement with the EMIT method was intermediate.