Diagnosing Cushing’s disease can be challenging. There are other conditions that can appear very similar and there is no one perfect test for this condition. The diagnostic process can therefore take a little while and involve a few different steps.
The first step is a general consultation with your vet. Your vet will take a detailed history and then examine your dog. If there are suspicious signs of Cushing’s disease, they will then recommend a general blood test and a urine sample as a second step.
Initial blood and urine testing
This second step will allow your vet to pick up on other potential causes of your dog’s problem, such as kidney disease, diabetes or liver disease. However, sometimes the results are still not completely clear, as Cushing’s disease causes an increase in the liver enzymes and blood cholesterol level, which diabetes and liver disease can also do. Depending on the results, your vet may recommend further, specific testing for Cushing’s disease.
There are three main tests to look for Cushing’s disease and they all have benefits and problems. Your vet will advise you on the most appropriate one for your dog. The following is just intended to help you to understand the limitations of each test.
ACTH stimulation test
This test involves taking an initial blood sample, injecting your dog with a synthetic ACTH (the hormone which stimulates steroid hormone production), then then repeating the blood sample 1 hour later.
A positive result can usually be trusted, however, some dogs with Cushing’s disease will have a negative ACTH stimulation test. If the suspicion of Cushing’s disease is still high, based on the other signs your dog is showing, your vet may recommend a further test.
Low dose dexamethasone suppression test
As with the previous test, this one involves taking an initial blood sample and then injecting your dog, but this time with a synthetic steroid. Further samples are then taken at 3 hours and 8 hours after the injection, so this test takes longer to perform.
This test can help to tell whether your dog has a problem in the pituitary gland, or in the adrenal glands. In this test, a negative result fairly reliably rules out Cushing’s disease, but some dogs without Cushing’s disease may have a positive result, so the test should only be used when the suspicion for the disease is high. In these cases, it is much more reliable.
Urine cortisol:creatinine ratio
This is a very easy test to perform. It simply involves collecting 3 urine samples in the home environment where your dog is not stressed. It is a good test for ruling out Cushing’s disease, but a positive result does not mean that your dog has the disease as many other things can cause positive results as well.
An ultrasound scan of the tummy (similar to that done for pregnant women) allows your vet to check for abnormalities such as liver disease, and also look at the size of the adrenal glands. If both adrenal glands are larger than normal, that suggests a tumour in the pituitary gland. If one is very large and the other very small, that shows that there is a tumour in the adrenal gland.
A CT scan is similar to an x-ray but takes pictures in cross-sections across your dog, allowing your vet to build a 3D picture of what is going on. It is better than ultrasound for looking at structures in the abdomen in larger dogs so can be used to look for adrenal tumours. It is also helpful to see if the adrenal tumour can be removed. A CT of the head may allow a pituitary tumour to be seen, but many of these are very tiny.
This is a blood test that involves directly testing the level of ACTH (which stimulates the adrenal glands to produce cortisol). This will be high in a dog with a pituitary tumour, and low in a dog with an adrenal tumour. The blood sample needs very special handling.