Abstract
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 (<2.0-9.0 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.
Highlights
Diagnostic procedures for the evaluation of Cushing’s syndrome (CS) include the following biochemical tests: 24hour urinary free cortisol (UFC) in two samples, serum cortisol measurement after oral administration of 1 mg of dexamethasone and salivary cortisol (Csal) determination in two samples collected at 11 pm based on the Endocrine Society Guidelines [1]
For 24-hour urinary free cortisol measurement, we found some limitations such as the quality of the sample associated with sampling difficulties and inconvenience for the patient
We evaluated the ability of cortisol determination at 11 pm to differentiate between healthy subjects and those with CS
Summary
Diagnostic procedures for the evaluation of Cushing’s syndrome (CS) include the following biochemical tests: 24hour urinary free cortisol (UFC) in two samples, serum cortisol measurement after oral administration of 1 mg of dexamethasone and salivary cortisol (Csal) determination in two samples collected at 11 pm based on the Endocrine Society Guidelines [1]. Measurement of steroids in the saliva offers several advantages over methods measuring total serum cortisol concentrations. These include a simple and noninvasive sampling procedure performed in a stress-free setting[8]. These advantages highlight the utility of saliva in certain populations such as pediatric patients, especially children with high weight, hypertension or osteoporosis, patients with psychiatric disorders or under high stress and subjects with severe renal failure [3, 9,10,11]. The aim of this study was to evaluate the analytical performance of such method (CLIA, Unicel 600 DxI Access® Beckman Coulter, Inc., U.S.A) and to find the cutoff value for the diagnosis of CS
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