Serum quinidine concentrations: comparison of fluorescence, gas-chromatographic, and gas-chromatographic/mass-spectrometric methods.

Serum quinidine concentrations were determined in patients on chronic therapeutic doses. Although results were higher by a protein precipitate-fluorescence method as compared to a specific extraction fluorescence method, there was substantial correlation between results by the two methods (r = 0.945, P less than 0.001). We established the specificity of the extraction method by a methylation gas-chromatographic method in which the base peak in the mass spectra of the methylated products of both quinidine and cinchonidine, the internal standard, was monitored. We conclude that the protein precipitate method should be discarded.