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Diagnostic Tests: Dexamethasone Suppression Test

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What is the Dexamethasone Suppression Test for Dogs?

The Dexamethasone Suppression Test is used to diagnose Cushing's Disease (Hyperadrenocorticism), or Cushing's Syndrome Due to Adrenal Tumor.

There are two versions of Dexamethasone suppression test, a low dose and a high dose DST.

The low dose dexamethasone suppression test (LDDST) is most commonly used for confirmation of the diagnosis, while the high dose dexamethasone suppression test (HDDST) is used to differentiate between HAC which is pituitary in origin, from those originating from adrenal tumors.

The low-dose dexamethasone suppression test is slightly more sensitive than the ACTH stimulation test but either of them cannot distinguish adrenal tumors from PDH. In normal dogs, dexamethasone administered in low doses inhibits pituitary secretion of ACTH and, in turn, cortisol secretion for 24-48 hours. Inadequate suppression of cortisol is found in 100% of dogs with adrenal tumors and in 95% of dogs with PDH. Using a 10-fold higher dose of dexa, most dogs with PDH will show decreased cortisol secretion, but the autonomous hypersecretion of cortisol by adrenal tumors will not be influenced by a higher dose of dexamethasone. This is useful to differentiate the primary cause of hyperadrenocorticism.

Dexamethasone Suppression Test for Diagnosing Dog Cushing's Disease

A low dose dexamethasone suppression test is a blood test that helps to diagnose Cushing's disease. Cushing's, or hyperadrenocorticism, occurs when the adrenal gland produce to much cortisol. It can cause increased water consumption and urination, skin problems, infections, and changes to the liver.

A low dose dexamethasone suppression test is a noninvasive series of blood tests. A initial sample is taken and then a small dose of the steroid dexamethasone is administered. Subsequent blood tests are taken 4 and 8 hours after dexamethasone is administered. The tests measure serum levels of cortisol as a response to the dexamethasone. If elevated, a diagnosis of Cushings is made.

Any test for Cushing's has drawbacks. It is possible to see false positive and false negative results with a LDDS test, so results should always be interpreted carefully. Despite the fact that it is a "test of choice" for confirmation of HAC, and that it has ~100% sensitivity, specificity of LDDST can be as low as 40%, and shouldn't be interpreted outside all other findings, and especially in otherwise systemically ill dogs. However, the LDDS is the most accurate of all the available tests, so is usually the preferred choice. The only exception is for dogs also suffering from diabetes, in which another test is preferred.

The LDDS test is noninvasive and safe. Your dog will have to stay at the veterinary hospital for the day in order to collect the samples, but there are no risks involved in the test.

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