Context: Although bedtime salivary cortisol measurement has been proposed as the optimal screening test for the diagnosis of Cushing's syndrome, its performance using commercially available assays has not been widely evaluated.

Objective: To compare radioimmunoassay (RIA) and tandem mass spectrometry (LC-MS/MS) measurement of salivary cortisol in obese subjects and healthy volunteers.

Design and Setting: Cross-sectional prospective study of outpatients

Subjects and Methods: We studied 261 obese subjects (186 female) with at least two additional features of Cushing's syndrome, and 60 healthy volunteers (30 female). Subjects provided split bedtime salivary samples for cortisol measurement by commercially available RIA and LC-MS/MS. Results were considered normal or abnormal based on the laboratory reference range. Subjects with abnormal results underwent evaluation for Cushing's syndrome.

Results: In paired samples, RIA gave a lower specificity than LC-MS/MS in obese subjects (86% vs. 94%, P=0.008) but not healthy volunteers (86% vs. 82%, P=0.71). Amongst subjects with at least one abnormal result, both values were abnormal in 44% (CI 26-62%) of obese and 75% (CI 33-96%) of healthy volunteers. In obese subjects, salivary cortisol concentrations were <4.0 to 643 ng/dl (<0.11 to 17.7 nmol/l; normal 100 ng/dl, 2.80 nmol/l) by LC-MS/MS, and <50 to 2800 ng/dl (1.4 to 77.3 nmol/l; normal 170 ng/dl, 4.7 nmol/l) by RIA. Cushing's syndrome was not diagnosed in any subject.

Conclusion: Salivary cortisol levels should not be used as the sole test to diagnose Cushing's syndrome if laboratory-provided reference ranges are used for diagnostic interpretation.