This study estimates diagnostic performance of late-night salivary cortisol (LNSC) as measured by automated electrochemiluminescence immunoassay (ECLIA), evaluates the clinical implication of two consecutive LNSC measurements, and compares its accuracy with enzyme-linked immunosorbent assay (ELISA) and serum cortisol after low-dose dexamethasone suppression test (DST) in obese and overweight patients referred for suspected Cushing's syndrome (CS). One hundred twenty three consecutive obese and overweight referred patients and 98 healthy volunteers provided two saliva samples collected at 23:00 using a Salivette (Sarstedt, Germany), assayed by ECLIA (Cobas e601) and ELISA. The patients underwent DST and were further evaluated until CS was pathologically confirmed (n = 45) or excluded. Diagnostic performance of LNSC was evaluated by receiver operating characteristic (ROC) analysis. The total areas under the curve (AUC) were calculated to compare the different tests. We found that a cut-off value of 9.4 nmol/l can differentiate CS among obese and overweight patients with sensitivity of 84.4 % (95% CI 71.2-92.2), specificity of 92.3 % (95% CI 84.2-96.4), and diagnostic odds ratio of 65.1 (95% CI 20.4-207.6). No difference was found between AUCs from the first, second, and the mean from the two LNSC measurements (ECLIA), LNSC (ELISA), or DST. The single LNSC (ECLIA) and DST improved the sensitivity and specificity for concordant results up to 100 and 97.4 %, respectively. In conclusion, due to its automation and its comparable diagnostic performance, ECLIA is preferable as a first-line LNSC screening test for CS. The initial use of single LNSC followed by DST provides better diagnostic performance for concordant results.

Nighttime salivary cortisol (NSC) has been suggested to be a useful diagnostic test for Cushings syndrome (CS). However, the reference range and cut-off value are assay-specific and discordant. The goal of this study was to assess the analytical performance of automated elecrochemiluminiscence immunoassay method (ECLIA) in CS. Ninety eight healthy volunteers and 123 obese patients including 45 proved to be CS provided salivary samples collected by them at 23:00 using Sallivette. Two hundred and five subjects collected salivary samples for two consecutive days and samples from 197 subjects were frozen to perform Enzyme-linked immunosorbent assay (ELISA). Obese patients underwent the 1-mg overnight dexamethasone suppression test (1-DST). CS was confirmed by the histologic diagnosis after surgical treatment or autopsy. The reference range for healthy volunteer has been set 0,5-9,4 nmol/l. Reproducibility was assessed in all subjects by a day-to-day variability and reflected by an intraclass correlation coefficient of 0,785. The cut-off value of 9,4 nmol/l has been suggested to differentiate CS among obese patients to achieve sensitivity of 84,4% (95%confidence interval 71,2-92,2%); specificity of 92,3% (95%CI 84,2-96,4%) and diagnostic odds ratio 65,1 (95% CI 20,4-207,6). Likelihood ratio positive was 11,0 (95% CI 5,0-23,9), with a likelihood ratio negative of 0,17 (95%CI 0,08-0,33). The comparison of the total areas under the ROC-curve for the measurement of NSC once, twice with mean level by ECLIA, the same samples by ELISA and 1-DST have not shown any statistically significant difference among the tests performance. Conclusion: Based on its remarkable reproducibility, easy noninvasive nature, automated assay and at least similar diagnostic performance, NSC measured by ECLIA on Cobas e601 is a preferable first-line screening test for CS.

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