Comparison of two low-dose dexamethasone suppression protocols as screening and discrimination tests in dogs with hyperadrenocorticism.

Two low-dose dexamethasone suppression test protocols were evaluated in 18 dogs with hyperadrenocorticism (14 dogs with pituitary-dependent hyperadrenocorticism [PDH] and 4 dogs with adrenocortical tumor) and in 5 healthy control dogs. Blood was obtained immediately before and 2, 4, 6, and 8 hours after IV administration of either 0.01 mg of dexamethasone sodium phosphate/kg of body weight or 0.015 mg of dexamethasone polyethylene glycol/kg. At 8 hours after dexamethasone administration, 18 of 18 (100%) dogs with hyperadrenocorticism given the sodium phosphate preparation and 16 of 18 (89%) affected dogs given the polyethylene glycol preparation failed to have suppression of plasma cortisol concentration (less than 1.4 micrograms/dl). Plasma cortisol concentration was suppressed to less than 1.4 micrograms/dl at 2, 4, and/or 6 hours after administration of either dexamethasone preparation in 5 of 14 dogs with PDH and to less than 50% of baseline cortisol concentration in 10 of 14 dogs with PDH. Suppression, as identified by these 2 criteria, was not observed at 2, 4, 6, or 8 hours after administration of either dexamethasone preparation in dogs with adrenocortical tumor. For both protocols, the 8-hour plasma cortisol concentration was suppressed to less than 1.4 micrograms/dl and to less than 50% of baseline in the 5 control dogs. Both protocols were comparable for use as screening tests in establishing a diagnosis of hyperadrenocorticism. Suppression of plasma cortisol concentration to less than 50% of baseline (or less than 1.4 micrograms/dl) during the test was consistent with diagnosis of PDH. Failure to have such suppression, however, was observed in dogs with PDH as well as in those with adrenocortical tumor.