Performance characteristics of five automated serum cortisol immunoassays

Background: Serum cortisol measurements are used to help diagnose conditions of cortisol deficiency and excess. Automated, nonisotopic immunoassays are used in most clinical laboratories. Methods: Linearity, imprecision, and method comparison studies for serum cortisol assays were performed with the Access®, Advia Centaur®, AxSYM®, Elecsys® 2010, and IMMULITE® 2000 analyzers. Results: All methods demonstrated acceptable linearity with the AxSYM method showing the least deviation from target values. The Elecsys 2010 method was the most precise for all three levels of quality control material. Method comparison studies demonstrated good concordance among methods except for two patient samples that gave results on the Elecsys 2010 that were significantly higher than the other methods. Differences in assay calibration were observed. Conclusions: Overall, all methods performed well. Additional calibration standardization efforts are required to truly harmonize the results from each assay. The Elecsys 2010 method is less specific than other automated methods based on two discordant patient results.