Definitively diagnosing canine hyperadrenocorticism (Cushing's disease) can be exceptionally difficult because nonadrenal illness can affect the test results. However, I think that the urine cortisol:creatinine ratio (UC:Cr) serves an invaluable role in ruling out canine Cushing's disease since a dog with a normal UC:Cr almost assuredly does not have Cushing's disease.