Validation of HPLC and Liquid-Liquid Extraction Methods for Warfarin Detection in Human Plasma and its Application to a Pharmacokinetics Study

A reversed-phase HPLC method to determine total plasma warfarin was developed and validated. Warfarin was extracted from human plasma using a two-step liquid-liquid extraction method. The residue was reconstituted with a phenylbutazone standard solution, which was used as the internal standard. The analytical column was a Purospher STAR RP-18e (4 x 4mm I.D., 5m particle size). The mobile phase consisted of acetonitrile: potassium dihydrogen orthophosphate buffer solution at pH 6.5 [30:70 (v/v)] with a flow rate of 1mL/min. Both warfarin and phenylbutazone were detected using a photodiode array detector. The lower limit of quantification was 100ng/mL, while the limit of detection was 20ng/mL. The linearity of the assay was good (r2=0.992) in the concentration range from 0.1 - 6.0µg/mL. The extraction recovery of warfarin was 93.53 ± 12.40%. Both the intraday and interday quality control assay for warfarin demonstrated good precision and accuracy, with all of the percentage coefficients of variation being less than 15%. Warfarin was stable in human plasma for up to three months of storage. The validated method was successfully applied to four human samples for a pharmacokinetics study.