Endocrine Abstracts

The diagnosis of Cushing’s syndrome (CS) is based primarily on diagnostic tests evaluating the cortisol response to dexamethasone suppression. Tests based on oral administration of dexamethasone may be compromised by poor compliance. We evaluated the diagnostic accuracy of a novel intravenous dexamethasone suppression test (IDST). The test is performed by intravenous (iv) bolus injection of 8 mg dexamethasone, with blood cortisol determinations made before injection, then hourly during the first 6 h and finally at 24 h. ACTH is measured prior to dexamethasone injection and at 6 and 24 h following injection. We performed a retrospective analysis of patients studied for suspected CS in Hadassah, between 1994–2004. The study included 101 patients: 54 patients with pituitary CS, 22 with adrenal CS, 4 with ectopic ACTH CS (EAS) and 24 in whom the diagnosis of CS was excluded. Patients without CS showed rapid suppression of cortisol and ACTH that persisted for 24 hours. Patients with pituitary CS showed suppression of cortisol and ACTH levels at 6 hours with subsequent escape at 24 hours. Patients with adrenal CS or with EAS failed to suppress cortisol or ACTH levels. Using 60% suppression of blood cortisol at 24 h as the cutoff for the diagnosis of CS, IDST had 94% sensitivity, 95% specificity and 98% positive predictive value (PPV) for the diagnosis of CS. Similar results were obtained by using a cortisol level of 200 nmol/l at 24 hours as the cutoff for the diagnosis of CS. Adding the criteria of ACTH levels >4 pmol/l at 24 hours, the PPV of the IDST increased to 100%. Conclusions: IDST is a reliable, simple and accurate test for diagnosing hypercortisolism. Measuring cortisol levels before and 24 h after 8 mg i.v. dexamethasone administration is required to adequately diagnose patients with CS. ACTH levels at 24 h may increase the test’s PPV. The sensitivity, specificity and PPV of the IDST are equal or higher than those reported for other commonly used non-invasive tests. Further studies are required to determine if IDST can discriminate effectively between pituitary disease and EAS.