Interpretation of laboratory tests for canine Cushing's syndrome

Hypercortisolism (HC) is a common disease in dogs. This article will review the laboratory tests that are available for diagnosis of HC and laboratory tests for differentiating between causes of HC. An emphasis will be made on the clinical process that leads to the decision to perform those tests and common misconceptions and issues that arise when performing them. To choose between the adrenocorticotropic hormone (ACTH)-stimulation test and the low-dose dexamethasone suppression test (LDDST), the advantages and disadvantages of both tests should be considered, as well as the clinical presentation. If the index of suspicion of HC is high and other diseases have been appropriately ruled out, the specificity of the ACTH stimulation test is reasonably high with an expected high positive predictive value. Because of the low sensitivity, a negative result in the ACTH stimulation test should not be used to rule out the diagnosis of HC. The LDDST is more sensitive but also less specific and affected more by stress. A positive result on the urine cortisol:creatinine ratio does not help to differentiate HC from other diseases. A negative result on the urine cortisol:creatinine ratio indicates that the diagnosis of HC is very unlikely. The LDDST is useful in differentiating pituitary-dependent HC from an adrenal tumor in about two thirds of all dogs with HC. Differentiation of HC from diabetes mellitus, liver diseases, and hypothyroidism cannot be based solely on endocrine tests. Clinical signs, imaging studies, histopathology, and response to treatment should all be considered.