ASSAY OF CORTISOL WITH A RADIOIMMUNOASSAY METHOD CALIBRATED BY ISOTOPE-DILUTION MASS-SPECTROMETRY - A NORDIC COLLABORATIVE STUDY

  • 1 January 1983
    • journal article
    • research article
    • Vol. 43  (5) , 433-437
Abstract
A reference method for serum cortisol, based on isotope dilution-mass spectrometry (ID-MS), was compared with a modified commercial RIA [radioimmunoassay] method. The modification solely concerned the calibration standards used in the RIA method. These were replaced by a series of human serum samples in which the concentration of cortisol was determined by the reference ID-MS method. The samples were selected so as to cover the whole range of the standard curve. Serum samples from healthy, untreated subjects with cortisol concentrations 270-1134 nmol/l were analyzed with the ID-MS calibrated RIA method in 4 laboratories, 1 in each of the 4 Nordic countries. The results from 2 of the 4 laboratories did not differ significantly from the ID-MS values (P > 0.05), while the results from the other 2 laboratories differed slightly. The mean values based on results from all 4 laboratories were almost identical with the values obtained with the reference method. In addition, serum samples from 11 patients with endocrine disorders with cortisol concentrations 31-916 nmol/l were analyzed in 3 of the 4 laboratories. In 3 of the samples significant differences were observed between the values obtained with the ID-MS and the ID-MS calibrated RIA method. The value obtained with the ID-MS calibrated RIA was however always more accurate than the corresponding value obtained with RIA with the use of a commercial calibration standard. Advantages of the present model in increasing the accuracy of routine analysis by RIA are discussed. The possibility is further discussed that the ID-MS calibrated RIA method may be used as a secondary reference method.