Urine Cortisol / Creatinine Ratio Test
This is considered a screening test, this test cannot diagnose Cushing’s Disease but it can rule it out. A urine sample is collected and examined for relative amounts of cortisol versus a normally excreted protein metabolite, creatinine. The greater the raio, the higher the cortisol level. High cortisol in the urine is suggestive of high cortisol in the bloodstream.
Many conditions other than Cushing’s disease can cause false positives so this test is not considered diagnostic. However, if the cortisol/creatinine ratio is okay, your pet is not likely to be Cushingoid, so this is often used as a good screening test.
ACTH Stimulation Test
To begin an ACTH Stimulation Test a blood sample is taken to determine a baseline for your pet. Your pet will then be given an injection of ACTH, the hormone which stimulates the adrenals to release cortisol. One to two hours later, blood cortisol levels are measured. Dog’s with Cushing’s disease have a constantly overworked, overproducing set of adrenals, the cortisol reserves are much greater, and the Cushingoid dog will be able to respond to the ACTH with greatly elevated cortisol output. This test doesn’t diferentiate between forms of Cushing’s (adrenal vs. pituitary).
This test is considered diagnostic 80 – 95% of the time. Half of the adrenal tumors will not respond to this test, and the 15-20% of dogs with pituitary tumors will not respond. This test is used as a monitoring test for dogs who are being treated for Cusings disease with Lysodren.
Low Dose Dexamethasone Suppression Test
This test has the ability to be used as a diagnostic test 90% of the time. A fasted dog has a blood sample taken as a baseline in the morning. A small amount of dexamethasone, a synthetic glucocorticoid, is injected and follow-up blood samples are taken 4 hours and 8 hours later.
A normal dog’s body will perceive the presence of dexamethasone and suppress cortisol output through the test. Cushingoid dogs will not suppress blood cortisol in response to the dexamethasone injection, because their feedback mechanisms are not working properly.
This test does not differentiate between the two forms of Cushing’s disease (adrenal vs. pituitary), although it may be suggestive. Dogs who suppress at 4 hours and rebound at 8 hours usually have pituitary tumors.