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.