NOCTURNAL 8MG DEXAMETHASONE SUPPRESSION TEST - A PRACTICAL AND ACCURATE TEST FOR IDENTIFICATION OF THE CAUSE OF ENDOGENOUS CUSHINGS-SYNDROME

The efficacy of a nocturnal 8mg dexamethasone suppression test (noctur nal DST) was compared with that of the standard high-dose dexamethason e suppression test (standard DST) in identifying the cause of endogeno us Cushing's syndrome in 10 proven cases with Cushing's disease, 20 wi th adrenal tumours, and one with ectopic ACTH syndrome. The nocturnal test compared serum cortisol concentration at 8am before and after adm inistration of a single dose of 8mg dexamethasone at 11pm. Suppression of serum cortisol level to <50% of the baseline value indicated a dia gnosis of Cushing's disease, while a lack of suppression below that li mit indicated one of the other two causes of Cushing's syndrome: gluco corticoid-secreting adrenal tumour or ectopic ACTH syndrome. The noctu rnal DST had a sensitivity of 90%, a specificity of 100%, an accuracy of 96.8%, a positive predictive value of 100%, and a negative predicti ve value of 95.5%. These values are comparable to the efficacy of the standard DST in distinguishing Cushing's disease from glucocorticoid-s ecreting adrenocortical tumour or ectopic ACTH syndrome. Furthermore, this rapid test does not require hospitalisation or urine collection l ike the standard DST. The nocturnal 8mg dexamethasone suppression test is practical, fairly reliable, and an effective alternative with whic h to identify the cause of endogenous Cushing's syndrome.