Diagnosing Cushing’s disease can be difficult, as there is no single definitive lab test and many conditions may present along with the disease, complicating the diagnosis. A successful identification of Cushing’s will depend on the owner’s vigilance and description of changes in the dog’s habits and body, along with an analysis of all the symptoms by a veterinarian.
After a physical examination of your dog, checking for an enlarged liver and examining any skin complaints, the veterinarian will likely send urine and blood to the lab for testing. Abnormalities in the blood that may indicate Cushing’s include increased SAP (serum alkaline phosphatase), increased ALT, high cholesterol, high blood sugar, and lower BUN (blood urea nitrogen). Diabetes often co-presents with Cushing’s.
If your dog is otherwise healthy, a urine test measuring the ratio of cortisol to creatinine can easily indicate Cushing’s disease, but this test may also return positive results if your dog has additional, different conditions. This urine sample must be obtained by the owner at home when the dog is relaxed- the stress of a car journey or fear of the veterinarian's office can cause stress-released cortisol to artificially inflate the cortisol to creatinine ratio.
Once Cushing’s disease is suspected, there are a number of tests that can be performed to confirm the diagnosis and shed light on the source of the condition (i.e. pituitary vs adrenal problem). An ACTH stimulation test is conducted by administering artificial ACTH and measuring the response of the adrenal glands. Elevated response from the adrenal glands disproportionate to the amount of ACTH administered can indicate Cushing’s disease, and implicates the adrenal glands (or over-treatment with steroids) as the cause. An LDDS is less sensitive (often giving false positives if the dog has a different illness) but rarely gives false negatives. LDDS involves administering a pituitary-suppressing compound and measuring how long it takes the pituitary to “escape” the suppression and continue producing ACTH. A 3-4 hour window where ACTH production is low, followed by resumption of production at around 8 hours strongly suggests pituitary-dependent Cushing’s.
Once Cushing’s has been tentatively diagnosed, the next step is medical imaging to determine whether a tumor or unusual damage may be causing the problem. To examine the pituitary gland, MRI is the machine of choice. For the adrenal glands, ultrasound is preferred, and likely can be used in your veterinarian's office upon suspecting Cushing’s, as it is easily portable and noninvasive.