Once there is some reason to suspect Cushing's disease based on the history, physical examination, and initial laboratory testing, it is necessary to do specific testing to confirm it. There are several options. We will begin with dogs and then discuss cats.

Canine Cushing's Disease Testing

The Low Dose Dexamethasone Suppression Test
Usually takes 8 hours in the hospital
Dexamethasone is a cortisone-type hormone that is used therapeutically for numerous conditions. When given dexamethasone, the dog's pituitary gland will perceive that there is a steroid and shut off its stimulatory message to the adrenal glands. In the normal animal, this means that a drop in blood cortisol level will be seen 8 hours after a tiny dose of dexamethasone is given intravenously.

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If either the pituitary tumor or the adrenal tumor form of Cushing's disease is present, the pituitary is not responsive to the presence of dexamethasone and continues to produce its stimulatory message regardless.  As a result, no drop in cortisol level is seen at the end of eight hours.

To Run this Test
Ideally the low dose dexamethasone suppression test is run in the morning. A baseline cortisol level is measured, a low dose of dexamethasone is given intravenously, and blood samples are checked again in 8 hours. Sometimes a 4-hour sample is also drawn as the pattern of suppression over the entire 8 hours may help  classify the type of Cushing's disease. The pet will require at least 8 hours in the hospital.

The ACTH Stim Test
Requires one to two hours in the hospital
Central to the concept of Cushing’s disease is the over-production of cortisol. It follows then that the adrenal glands of the Cushing’s patient would possess large amounts of stored cortisol hormone. So how do we detect these unusually large amounts of stored hormone vs. normal? Here's how: we give a dose of ACTH (adrenocorticotrophic hormone), which is the hormone the pituitary gland uses to stimulate release of cortisol. Basically, we give a dose of ACTH and see if we get a normal cortisol release or an extra-large one. A baseline cortisol level is drawn, the ACTH is given as a shot, and then in 1-2 hours a second cortisol level is measured to compare. In this way, the adrenal gland's potential to release cortisol is measured.

To Run this Test
Ideally, the patient is fasted overnight and the test is performed in the morning between 8 a.m. and 10 a.m. A blood sample is drawn, a dose of ACTH is given, and two hours later a second blood sample is drawn.

When Would we Run this Test?
Given that the Low Dose Dexamethasone Suppression test is more accurate, when might we run this test instead? It turns out that this is the only test that can be used if Cushing's disease from overuse of steroid medications is suspected; the low dose dexamethasone suppression test does not work if medications have suppressed the patient's natural system. The ACTH Stim test is also crucial in monitoring Cushing's patients during their treatment. This test is more specific than the low dose dexamethasone suppression test meaning that the strongly positive dogs definitely have Cushing's disease (false positives are unusual) but since it is not a very sensitive test, the University of California Veterinary School no longer recommends this test to determine whether or not a dog has Cushing's disease. This is, as mentioned, the test that is used to monitor the treatment of Cushing's disease, however, and a dog being treated for Cushing's disease will likely look forward to many ACTH Stimulation tests ahead.

The Urine Cortisol/Creatinine Ratio
This is a screening test for Cushing's disease; a positive test here does NOT confirm Cushing's syndrome but a negative test DOES rule it out. In this test, a single urine sample is collected to determine the relative amounts of cortisol and creatinine (creatinine is a protein metabolite that is constantly excreted in urine). If there is a high ratio - a relatively high amount of cortisol being excreted - further testing is in order. Ideally for this test the urine sample is collected at home so that cortisol secreted in response to the stress of visiting the vet's office does not interfere with results.

Feline Cushing's Disease Testing

The Low Dose Dexamethasone Suppression Test
The testing situation is somewhat similar in cats, though the ACTH Stim test is generally not considered useful.

Photo by MarVistaVet

This test is performed similar to the way it is performed with dogs; however, the dose of dexamethasone required to test a cat is substantially higher as cats are more resistant to the effects of steroids. The test still requires eight hours in the hospital and the cat should be relaxed and kept quiet during the stay. If the cat is the type to experience a great deal of stress in visiting the vet's office, consider the urine cortisol:creatinine ratio.

The Urine Cortisol/Creatinine Ratio

This test provides an excellent screening test for Cushing's disease in cats and can be performed to include a high dose dexamethasone supression test to further categorize the Cushing's disease so as to best determine treatment. The owner should bring urine samples collected first thing in the morning on three consecutive mornings. Having the sample collected at home removes the possibility of interference from vet visit-induced stress. The first two samples are used to determine if the cat has Cushing's disease and the ratios are averaged. On the second day (after the second sample is brought in) dexamethasone pills are given to the cat at home and the sample from the third day is used to determine the type of Cushing's disease the cat has.

Collecting a cat's urine at home is easier than it sounds. The cat is confined overnight and in the morning is allowed access to the litter box. The easiest way to collect the urine is to place a sheet of cellophane over the box. The cat will runkle this up a bit scratching around but only a small amount need be caught on the surface of the cellophane. A syringe can be used to suck up the sample and store it for transport to the veterinary clinic.

No blood drawing is needed and the cat does not have to actually come to the vet's office for the test. The problem is that urine cortisol:creatinine ratios are possibly too sensitive and may generate false positives. For this reason, the low dose dexamethasone suppression test is listed by most experts as the screening test of choice for cats.

It is important to note a diagnosis of Cushing's disease but it is equally important to consider that successful treatment of Cushing's is about controlling symptoms. If the symptoms do not warrant control, then treatment should be postponed until they are more problematic. After it has been determined that a pet has it, it is important to determine the type of Cushing's disease (adrenal vs. pituitary tumor). The next section on classifying the type of Cushing's reviews how this is done. Treatment options and prognosis are highly dependent on the type of Cushing's disease the pet has.