EFFECTS OF DISEASE ON THE RESULTS OF DIAGNOSTIC-TESTS FOR USE IN DETECTING HYPERADRENOCORTICISM IN DOGS

The purpose of the study reported here was to assess 3 commonly used s creening tests for hyperadrenocorticism (low-dose dexamethasone suppre ssion test, ACTH stimulation test, and urinary cortisol:creatinine rat io) in dogs with various diseases other that those of the adrenal glan ds (nonadrenal diseases). A group of 100 dogs was studied: 59 dogs wit h nonadrenal disease, 21 clinically normal dos, and 20 dogs with pitui tary-dependent hyperadrenocorticism. Of 59 dogs with nonadrenal diseas e, 20 (34%) had high baseline cortisol concentration (greater than ref erence range limits), and 22 (38%) and 33 (56%) had inadequate serum c ortisol suppression at 4 and 8 hours, respectively, after administrati on of a low dose of dexamethasone. Compared with clinically normal dog s, dogs with nonadrenal disease had significantly (P < 0.05) higher me an serum cortisol concentration at 4 and 8 hours after administration of a low dose of dexamethasone; however, significant differences were not detected between the mean cortisol concentration at 8 hours after administration of dogs with nonadrenal disease and for dogs with hyper adrenocorticism. After ACTH stimulation, only 8 fo 59 (14%) dogs with nonadrenal disease had high serum cortisol concentrations. Significant differences did not exist after ACTH stimulation between mean cortiso l concentration of clinically normal dogs and that of dogs with nonadr enal disease. Of 59 dogs with nonadrenal disease, 45 (76%) had a high urinary cortisol:creatinine ratio. When compared with clinically norma l dogs, dogs with nonadrenal disease had a significantly higher mean u rinary cortisol:creatinine ratio, but significantly differences did no t exist between the mean urinary cortisol:creatinine ratio of dogs wit h nonadrenal disease and that of dogs with hyperadrenocorticism. The c alculated sensitivity for each diagnostic test used on the 20 dogs wit h hyperadrenocorticism in this study ranged from 0.70 to 1.0, with the ACTH stimulation test having the lowest sensitivity and the low-dose dexamethasone suppression test having the highest sensitivity. In dogs with nonadrenal disease, the ACTH stimulation test had the highest sp ecificity low-dose dexamethasone suppression test (0.44) and urinary c ortisol:creatinine ratio (0.24) were lower. The ACTH stimulation test also had the highest positive predictive value (0.67), whereas this va lue was lower for the low-dose dexamethasone suppression test (0.38) a nd urinary cortisol:creatinine ratio (0.25). Analysis of results of th is study indicated that many dogs with nonadrenal diseases have false- positive test results for hyperadrenocorticism when tested with the co mmonly used pituitary-adrenal function tests. Because false-positive t est results were observed for all of the commonly used screening test, a definitive diagnosis of hyperadrenocorticism should never be made s olely on the bassi of results of 1 or more of these screening tests, e specially in dogs without classic clinical signs of hyperadrenocortici sm or in dogs known to have nonadrenal disease.