Determination of naproxen and its desmethyl metabolite in human plasma or serum by high performance liquid chromatography.

1. A liquid chromatographic method has been developed for the simultaneous determination of the anti-inflammatory agent naproxen and its metabolite, 6-0-desmethyl-naproxen, in human plasma or serum. 2. After addition of p-chlorowarfarin as the internal stardard, plasma is acidified and extracted with either chloroform (for assay of naproxen only) or ethyl acetate (for assay of naproxen and desmethyl-naproxen), the organic solvent is evaporated, the residue is dissolved in acetonitrile and injected onto a reverse phase column coupled with an ultra-violet absorbance detector. 3. Drug and metabolite can be detected readily in concentrations above 2 micrograms per ml in 0.5 ml samples. Salicylic acid and its metabolites and a number of other durgs were found not to interfere with the assay.