Dexamethasone suppression test

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The dexamethasone suppression test (LDDS - low dexamethasone test) is an adjunct test used for diagnosis of cushing's syndrome in cats.

The LDDS test is used but the dexamethasone that is injected needs to be given at a higher dose. This test, when used in conjunction with the ACTH stimulation test, is one of the best ways to diagnose Cushing's in the cat.

  • Advantages - The low-dose dexamethasone suppression test is more reliable than the ACTH stimulation test in confirming hyperadrenocorticism, since the results are diagnostic in all adrenal-dependent cases and in 90 to 95 per cent of dogs with pituitary-dependent hyperadrenocorticism.
  • Disadvantages - The low-dose dexamethasone suppression test is not as accurate as the ACTH stimulation test for the detection of iatrogenic hyperadrenocorticism. The test is also affected by more variables than the ACTH stimulation test, takes 8 hours to complete and does not provide pre-treatment information that may used in monitoring the effects of mitotane therapy. The low-dose dexamethasone suppression test does not reliably differentiate pituitary-dependent from adrenal-dependent hyperadrenocorticism.
  • Interpretation - Interpretation of the results of a low-dose dexamethasone suppression test must be based on the laboratory's normal range for the dose and preparation of dexamethasone administered. If the dose of dexamethasone fails to adequately suppress circulating cortisol concentrations in a dog with compatible clinical signs, a diagnosis of hyperadrenocorticism is confirmed. While basal and 8-hour post-dexamethasone samples are most important for interpretation of the test, one or more samples taken at intermediate times (for example, 2, 4, or 6 hours) during the test period may also prove helpful. If a plasma cortisol concentration determined two to six hours after dexamethasone injection is suppressed normally or near-normally (to below 40 mmol/l), while the 8-hour sample shows escape from cortisol suppression, then a diagnosis of pituitary-dependent hyperadrenocorticism is indicated (Peterson, 1984).
Serum cortisol levels high and low dexamethasone suppression tests. In a healthy individual, the administration of a low dose of oral dexamethasone (0.5mg 6-hourly) will lead to suppression of cortisol levels below 50nmol/L. No fall occurs in patients with ectopic adrenocorticotropic hormone (ACTH) syndrome, even with high doses of dexamethasone. This contrasts with the responses seen in patients with pituitary-dependent disease in whom inadequate suppression after low-dose administration of dexamethasone, and at least 50% suppression after administration of a high dose of dexamethasone (2mg 6-hourly) are characteristic.

The HDDS test to differentiate PD from AT has not been refined to the point that is of diagnostic value.

Test procedure

Note : this test is considered the test of choice in cats as an aid in confirming Cushing’s disease. The literature reveals 100% of cats with confirmed hyperadrenocorticism had an abnormal 8 hour postdexamethasone value and 95% had an abnormal 4 hour post-dexamethasone value. The remaining 5% having a borderline result in the 4 hour sample[1].

  • Protocol

Patient preparation. The animal should preferably be fasted before collection. Pituitary-adrenal function testing is best performed in a healthy, non-stressed animal. Given the circumstances leading to testing, this is rarely achievable, however, minimizing stress/illness is nonetheless recommended. Note also therapy with corticosteroids can alter the pituitary-adrenal axis, hence where possible should be avoided, especially in the previous 48-72 hours.

  1. Collect blood into a plain or SST for a baseline cortisol and clearly label as the resting or 0 hr sample.
  2. Administer 0.1 mg/kg of dexamethasone IV (Note this is in contrast to canine testing, where 0.01 mg/kg is used).
  3. Collect post-dexamethasone blood into a plain or SST and again clearly label with appropriate time. Samples should be collected at both 4 hours and 8 hours post-dexamethasone administration. Serum cortisol should be < 30 nmol/L. Normal suppression rules out diagnosis. Levels > 30 nmol/L are suggestive of Cushing's Syndrome.
  4. Submit all three samples (resting, 4 hour and 8 hour post-dexamethasone) together to the laboratory for testing.

References

  1. Feldman EC, Nelson RW (2004) Canine and Feline Endocrinology and Reproduction, 3rd Edition
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