The use of the high-dose dexamethasone suppression test (HDDS test) on blood is recommended when the low-dose dexamethasone suppression test (LDDS test) has not been able to differentiate adrenal-dependent hypercorticism (ADH) from pituitary-dependent hypercorticism (PDH). As a matter of fact, a pituitary tumour that does not respond to a dose of 0.01 mg/dl (low dose) of dexamethasone might respond to higher doses while, in patients with adrenal tumours, there will be no suppression of blood cortisol regardless of the dose used.
The HDDS test is performed by measuring blood cortisol levels before and 3-4 hours after an intravenous administration of 0.1 mg/kg of dexamethasone (a dose 10 times higher than that used in the LDDS test). It should be kept in mind that even with the HDDS test, there is no suppression of cortisol in around 15-25% of the cases of PDH, which, therefore, cannot be distinguished from cases of ADH.