Comparative study of midnight salivary cortisol measurements: Impact on clinical interpretation

Despite the other first-line tests for investigating hypercortisolism, saliva analysis is rather easy to collect and it does not require hospitalization. Indeed, the patient can easily collect the saliva sample from home by using a specific device, assuring the good conditions of the specimen until arrival to the laboratory. Also, in crucial categories of patients, such as patients with renal failure and pregnant women, the midnight salivary cortisol proved to be more robust and reliable analysis. In this work, we compared the immunoanalysis methods commercialized for the salivary cortisol measurements, the Elecsys Cortisol II assay on COBAS 8000 (ROCHE Diagnostics) with a LC-MS/MS method already developed in our laboratory for UFC measurements. Analytical performance (imprecision, linearity and limit of detection) was evaluated. Therefore, the LC-MS/MS method was investigated for detection of cortisol in saliva and particularly the sensitivity, the effect of the matrix and the null interferences with other steroids compared to immunoanalysis were taken into account. Consequently, a comparison of approximately 50 saliva samples from patients with suspicious of Cushing syndrome was performed on the two systems. The comparison with LCMS method allowed to show that the immunoanalysis presents similar results for the studied range of concentrations with no particular problems of saliva matrix effects. For the external controls, we observed a lower signal of our LC-MS/MS system probably due to the extraction systems of the different LCMS. The COBAS result in the laboratory was more similar to the pairwise target, although superior to the LCMS results.