Evaluation of the Performance of an Automated Chemiluminescent Method (Access Beckman Coulter) for Determination of Salivary Cortisol and Clinical Utility

The diagnostic procedure for the evaluation of Cushing’s syndrome is performed by any of the following biochemical tests: urine free cortisol, salivary cortisol at 11 pm and serum cortisol post 1 mg of dexamethasone. Collection of saliva samples is simple and noninvasive, thus being a method of choice for the evaluation of risk populations. The aim of this work is to analyze the performance of an automated chemiluminescent method for measurement of salivary cortisol at 11 pm according to the new quality guidelines and assess its clinical utility. Cortisol levels were measured in samples obtained by passive drooling from 32 healthy subjects and 9 patients with Cushing’s syndrome. Matrix effect, linearity, limit of blank, limit of quantitation, recovery and diagnostic performance were assessed. The Unicel 600 DXI Access Beckman Coulter chemiluminescent automated analyzer was used. The standard curve provided by the manufacturer was adapted to measure cortisol concentrations in saliva. Matrix effect: equation of the curve using salivary matrix: y=-1.824x+3.491 (95% CI=-2.068 to -1.582) vs. Equation of the curve using diluent matrix: y=-1.833x+3.394 (95% CI=-1.961 to -1.704). There is overlapping of both curves. Linearity: linear assay between 1.8 nmol/L and 108.0 nmol/L. Limit of blank: 0.1 nmol/L. Limit of quantitation: 1.8 nmol/L (TAE of 25%). Recovery: standard cortisol solution concentration 5 nmol/L: 102%; 10 nmol/L: 107%; 40nmol/L: 115%. Diagnostic performance: median and ranges in healthy subjects: 2.0 nmol/L (u003c2.0-9.0 nmol/L); Cushing’s syndrome: 30.3 nmol/L (15.4-61.0 nmol/L). ROC curve cutoff value: 9.0 nmol/L (100% Specificity; 100% Sensitivity; AUC=1.00). The method used provides excellent analytical performance for cortisol measurement in saliva at 11 pm, which makes it a valuable biochemical tool both for screening populations at risk for Cushing’s syndrome and for the follow-up and diagnosis of this condition.