Nocturnal 8 mg dexamethasone suppression test: a practical and accurate test for identification of the cause of endogenous Cushing's syndrome.

The efficacy of a nocturnal 8 mg dexamethasone suppression test (nocturnal DST) was compared with that of the standard high-dose dexamethasone suppression test (standard DST) in identifying the cause of endogenous Cushing's syndrome in 10 proven cases with Cushing's disease, 20 with adrenal tumours, and one with ectopic ACTH syndrome. The nocturnal test compared serum cortisol concentration at 8 am before and after administration of a single dose of 8 mg dexamethasone at 11 pm. Suppression of serum cortisol level to < 50% of the baseline value indicated a diagnosis of Cushing's disease, while a lack of suppression below that limit indicated one of the other two causes of Cushing's syndrome: glucocorticoid-secreting adrenal tumour or ectopic ACTH syndrome. The nocturnal DST had a sensitivity of 90%, a specificity of 100%, an accuracy of 96.8%, a positive predictive value of 100%, and a negative predictive value of 95.5%. These values are comparable to the efficacy of the standard DST in distinguishing Cushing's disease from glucocorticoid-secreting adrenocortical tumour or ectopic ACTH syndrome. Furthermore, this rapid test does not require hospitalisation or urine collection like the standard DST. The nocturnal 8 mg dexamethasone suppression test is practical, fairly reliable, and an effective alternative with which to identify the cause of endogenous Cushing's syndrome.