Abstract 61: Diagnostic value of late-night salivary cortisol measured by Liquid Chromatography-Mass Spectrometry (LC-MS/MS) and electro-chemiluminescence assay (ECLIA) as a screening test for Cushing's syndrome

Background: Some of the signs and symptoms of Cushing's syndrome are non-specific and the diagnostic modalities are cumbersome or require hospital admission. Late-night salivary cortisol (LNSC) is an alternative to midnight serum cortisol sampling. But, most immunoassays have poor sensitivity and specificity at the low diagnostic cut-off, which is close to the lower limit of detection. Measurement of LNSC by liquid chromatography-mass spectrometry (LC-MS/MS) or highly sensitive electro-chemiluminescence assay (ECLIA) is an alternative. Objectives: The primary objective was to assess the diagnostic utility of LNSC measured using LC-MS/MS for the diagnosis of Cushing's syndrome and compare the values in obese and healthy controls. The secondary objective was to compare the diagnostic performance of LNSC measurements by LC-MS/MS with the Roche II ECLIA for the diagnosis of Cushing's syndrome. Methodology: It was a cross-sectional study. Three groups of subjects were studied: group 1 –subjects with Cushing's syndrome diagnosed by two of the three screening tests being positive (midnight serum cortisol, 24 hr Urinary Cortisol and 1 mg overnight dexamethasone suppression test [ONDST]), group 2 –obese controls recruited from the Bariatric clinic, and group 3- normal weight controls recruited from the community. Patients with pre-existing psychiatric illnesses, chronic liver disease, and chronic kidney disease were excluded. After taking informed consent, thorough history including demography, previous illnesses, and drug history weretaken. All participants underwent routine anthropometric measurements and subjects in group 1 underwent biochemical evaluationfor Cushing's syndrome. Obese controls were tested with 8am serum Cortisol and 1 mg ONDST to exclude Cushing's syndrome. All subjects collected saliva at 11 PM on two consecutive nights using Salivette. Salivary cortisol was measured byLC-MS/MS and Roche II assay. Results: The mean age of the subjects with Cushing's syndrome was 32.25 ± 8.98 years. In group 2 and group 3, the mean age was 37.74 ± 11.80 years and 31.57 ± 7.04 years respectively. With a receiver operating characteristic (ROC) curve, an LNSC cut-off by LC-MS/MS of 0.054 mg/dl had a sensitivity of 88.4% and specificity of 70.6% for the diagnosis of Cushing's syndrome; and Roche II assay LNSC cut-off of 0.135 mg/dl had asensitivity of 95.2% and specificity of 93.2%. There was no significant day-to-day variability between LNSC values assayed by Roche II ECLIA or LC- MS/MS methods in all the three groups. Conclusion: We recommend measurement of LNSC by the Roche II ECLIA and suggest a cut-off of 0.135 mg /dl for the diagnosis of Cushing's syndrome.