Determination of valproic acid in plasma or serum by solid-phase column extraction and gas-liquid chromatography

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 microliters plasma or serum and gas-liquid chromatography (GLC) analysis, using 10% Carbowax 6000 on a Chromosorb WAW 80/100 column and alpha-methyl-alpha-ethylcaproic acid as an internal standard. The extraction efficiency for VPA at a concentration of 70 micrograms/ml is 98 +/- 4%. The calibration curve is linear in the range of 1.0-280 microliters/ml. The detection limit of VPA is 1.0 micrograms/ml. Syva enzyme multiplied immunoassay technique (EMIT) Free-Level filters were used in the analysis of free VPA from plasma. The described Bond Elut X GLC (BE X 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 X GLC analysis. The 44 sera involved were also analyzed by EMIT. In addition, BE X GLC was compared with the Abbott TDx system on an additional 21 specimens. Statistical agreement was excellent between BE X GLC and TDx, but poor between BE X GLC and liquid-liquid extraction X GLC. Agreement with the EMIT method was intermediate.