Which of the following tests is best for differentiating ACTH secreting pituitary tumour from Ectopic ACTH production?

24 hour urinary coisol levels are elevated in both subsets: ACTH secreting pituitary tumour and Ectopic ACTH secretion. Overnight Dexamethasone suppression test involves overnight 1mg dexamethasone tablet and measurement of 8am morning serum or urinary coisol. It is used to diagnose hypercoisolism but cannot identify etiology of Cushing syndrome. High dose dexamethasone suppression test involves giving 6 hourly dexamethasone 2mg, which leads to etiological diagnosis.( differentiates between ACTH secreting pituitary tumor from Ectopic ACTH ) Serum Coisol> 5mg/dl: Ectopic ACTH production.