Effects of disease on the results of diagnostic tests for use in detecting hyperadrenocorticism in dogs

Abstract

Summary The purpose of the study reported here was to assess 3 commonly used screening tests for hyperadrenocorticism (low-dose dexamethasone suppression test, acth stimulation test, and urinary cortisol:creatinine ratio) in dogs with various diseases other than those of the adrenal glands (nonadrenal diseases). A group of 100 dogs was studied: 59 dogs with nonadrenal disease, 21 clinically normal dogs, and 20 dogs with pituitary-dependent hyperadrenocorticism. Of 59 dogs with nonadrenal disease, 20 (34%) had high baseline cortisol concentration (greater than reference range limits), and 22 (38%) and 33 (56%) had inadequate serum cortisol suppression at 4 and 8 hours, respectively, after administration of a low dose of dexamethasone. Compared with clinically normal dogs, dogs with nonadrenal disease had significantly (P < 0.05) higher mean 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 for dogs with nonadrenal disease and for dogs with hyperadrenocorticism. After acth stimulation, only 8 of 59 (14%) dogs with nonadrenal disease had high serum cortisol concentrations. Significant differences did not exist after acth stimulation between mean cortisol concentration of clinically normal dogs and that of dogs with nonadrenal disease. Of 59 dogs with nonadrenal disease, 45 (76%) had a high urinary cortisol:creatinine ratio. When compared with clinically normal dogs, dogs with nonadrenal disease had a significantly higher mean urinary cortisol:creatinine ratio, but significant differences did not exist between the mean urinary cortisol:creatinine ratio of dogs with nonadrenal disease and that of dogs with hyperadrenocorticism. The calculated sensitivity for each diagnostic test used on the 20 dogs with 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 specificity (0.86) and efficiency (0.85), whereas the specificity of the low-dose dexamethasone suppression test (0.44) and urinary cortisol:creatinine ratio (0.24) were lower. The acth stimulation test also had the highest positive predictive value (0.67), whereas this value was lower for the low-dose dexamethasone suppression test (0.38) and urinary cortisol:creatinine ratio (0.25). Analysis of results of this study indicated that many dogs with nonadrenal disease have false-positive test results for hyperadrenocorticism when tested with the commonly used pituitary-adrenal function tests. Because false-positive test results were observed for all of the commonly used screening tests, a definitive diagnosis of hyperadrenocorticism should never be made solely on the basis of results of 1 or more of these screening tests, especially in dogs without classic clinical signs of hyperadrenocorticism or in dogs known to have nonadrenal disease.