An External Quality Control Survey (EQCS) for Serum Cortisol

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Abstract
The external quality control survey (EQCS) was based on those previously published from this department. Each participant received 20 [human] sera sent in dry-ice by express post or airmail, together with a detailed questionnaire requesting assay details and raw data from his own standard curve as well as from the 20 samples. The data returned was transferred to punched cards and processed by computer (Siemens 404/3) using a modular system of programs and sub-routines. The serum samples contained a hidden standard curve, samples for assay internal control and sera from suppression and stimulation tests. Possible cross-reacting substances were added to 2 sera. Each participant received a detailed computer print-out with an explanatory letter and set of histograms for each serum sample with which he could check his performance against other participants, in particular those using the same kit as himself. Eighty-seven laboratories received the 20 samples for cortisol assay, 64 laboratories returned data on the questionnaire sent, and of these, 49 sets of data could be fully worked out, the other 15 were only partly interpretable. Apart from laboratory-own methods, results from 9 different commercial kits were received. Twenty-one participants had an intra-assay coefficient of variation under 5%, 16 participants between 5 and 10%. The minimum coefficient of variation for the 3 sera measured was 0% and the maximum 86%. Coincidence of laboratory-own and hidden standard curves was only seen in 13 cases.