Determination of warfarin in human plasma by high performance liquid chromatography

A simple procedure for the determination of warfarin in plasma samples is described. The assay involves a single ethereal extraction of warfarin and its internal standard, propyl paraben, from an acidified sample. The extract is evaporated to dryness at 45 degrees C and the residue is redissolved in methanol (50 microliters). A 25-microliters aliquot is injected into the liquid chromatograph and eluted with acetonitrile - 10 mM phosphate buffer at pH 3.5 (48:52, v/v) on a C8 pre-column linked to a 5-microns C8 reversed-phase column. The flow rate was set at 1.5 ml min-1 and UV detector at 215 nm. The method has been used to determine plasma concentrations of warfarin in Chinese patients with recent atrial fibrillation scheduled for electrocardioversion.