Liquid chromatographic assay of warfarin: similarity of warfarin half-lives in human subjects.

A high pressure liquid chromatographic assay was developed to measure warfarin concentrations in biological fluids. Twelve healthy, unrelated volunteers received a single oral dose of warfarin (0.75 mg per kilogram of body weight). The mean plasma warfarin half-life was 36.3 +/- 3.5 hours by liquid chromatography but 55.9 +/- 8.4 hours by a currently used fluorimetric assay that fails to separate warfarin from its metabolites. Interindividual variation was greater and each half-life longer by the fluorimetric than by the chromatographic procedure. Warfarin shows less interindividual variation than that observed for other drugs primarily metabolized by hepatic microsomal mixed function oxidases. Advantages of specificity, rapidity, sensitivity, accuracy, and simplicity recommend liquid chromatography in the development of other drug assays.