Candidate reference method for determining serum cortisol based on isotope dilution-gas chromatography/mass spectrometry using heptafluorobutyrilation as derivatization method.

We present a new candidate reference method for determining cortisol in serum. The method is based on isotope dilution-gas chromatography/mass spectrometry and makes use of derivatization with heptafluorobutyric anhydride and selected ion monitoring at m/z 489 and 491. A detection limit of 0.08 pmol (30 pg) was achieved. Twenty-four structurally related steroids were tested for interference and found negative. Verification of the analytical quality specifications was done from measurement in two laboratories of the certified reference materials 192 and 193 of the Bureau Communautaire de Référence and a number of commercial quality control materials. The maximum systematic error was estimated to be 1.0% and the mean imprecision was 1.0%. The total error was lower than 2.05%. The method was applied for target-setting in external quality assessment and internal accuracy control and for measurement of patient samples.