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 i n serum. The method is based on isotope dilution-gas chromatography/ma ss spectrometry and makes use of derivatization with heptafluorobutyri c anhydride and selected ion monitoring at mit 489 and 491, A detectio n limit of 0.08 pmol (30 pg) was achieved. Twenty-four structurally re lated steroids were tested for interference and found negative, Verifi cation of the analytical quality specifications was done from measurem ent in two laboratories of the certified reference materials 192 and 1 93 of the Bureau Communautaire de Reference 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 low er than 2.05%. The method was applied for target-setting in external q uality assessment and internal accuracy control and for measurement of patient samples. (C) 1996 Academic Press, Inc.