A SINGLE SLEEPING MIDNIGHT CORTISOL HAS 100-PERCENT SENSITIVITY FOR THE DIAGNOSIS OF CUSHINGS-SYNDROME

OBJECTIVE The diagnosis of Gushing's syndrome remains a major challeng e in clinical endocrinology. Various screening tests are commonly used to support a biochemical diagnosis in the context of clinical suspici on. The aim of this study was to compare the sensitivity in the diagno sis of Gushing's syndrome of a single inpatient sleeping midnight cort isol to a standard 48-hour in-patient low-dose dexamethasone suppressi on test (LDDST) during the same admission. DESIGN A retrospective anal ysis was performed on 150 patients investigated in our department betw een the years 1970 and 1994 with a confirmed diagnosis of Gushing's sy ndrome. PATIENTS One hundred and fifty patients with a diagnosis of Gu shing's syndrome were analysed: 110 with Gushing's disease; 12 with tu mours with ectopic ACTH secretion; 8 with ACTH dependent Gushing's syn drome of so far undetermined origin; 17 with cortisol secreting adrena l tumours; 3 with adrenocortical nodular hyperplasia. Twenty normal vo lunteers and nine patients with non-endocrine conditions were also inv estigated as controls. MEASUREMENTS Plasma cortisol was measured by ra dioimmunoassay (RIA) in the 122 patients presenting after 1980, and by fluorimetry prior to this date. RESULTS In all the control subjects t he sleeping midnight cortisol was < 50 nmol/l, below the lowest standa rd of the routine in-house RIA. In every patient with Cushing's syndro me the sleeping midnight cortisol was detectable with a value greater than 50 nmol/l, with a range of 70-2000 nmol/l. In contrast, in three cases, all of whom had proven Gushing's disease on histology, there wa s uncharacteristic complete suppression of plasma cortisol to < 50 nmo l/l following the LDDST. CONCLUSION In this series of 150 cases, a sin gle inpatient sleeping midnight cortisol above 50 nmol/l had a 100% se nsitivity for the diagnosis of Gushing's syndrome, clearly different f rom normal subjects. In contrast, the low-dose dexamethasone suppressi on test had a sensitivity of 98% even when the drug was administered a s an in-patient. We recommend that a low-dose dexamethasone suppressio n test should not be used alone for confirmation of Gushing's syndrome since it may miss 2% of cases.