Detecting patients with Cushing's syndrome: The importance of initial test selection

Background and objective: The recommended initial tests for suspected Cushing's syndrome are latenight salivary cortisol (LNSC), 24-hour urinary free cortisol (UFC) and the 1 mg overnight dexamethasone suppression test (ONDST). These tests have higher sensitivity and specificity than serum cortisol. The aim of this study was to determine the relative frequency of these requested tests in primary care. Methods: Initial test selection for investigation of Cushing's syndrome was audited by reviewing pathology request forms for cortisol tests made to a major community-based laboratory in 2019. Those with hypertension or adrenal incidentaloma as the documented indication for testing were included. Results: In 214 of 272 cases (78.7%; 95% confidence interval: 73.2%, 83.3%) initial testing was by measurement of serum cortisol alone. Discussion: The relatively infrequent selection of the higher sensitivity tests (ONDST, UFC and LNSC) for investigation of suspected Cushing's syndrome signifies a risk of delayed or missed diagnosis, with important implications for morbidity and mortality.