Cushing’s Disease Diagnosed with Low Dose Dexamethasone Suppression Test
Julie Piccione, DVM, MS, DACVP
A 13-year-old female spayed Beagle was presented to its veterinarian for increased urination. On physical exam, the veterinarian noted a pot-bellied appearance and thinning hair coat. A minimum data base (CBC, Chemistry, and urinalysis) was performed in-house and revealed only a markedly increased ALKP activity, low lymphocyte count, and a low urine specific gravity (specific values were not provided). The veterinarian suspected Cushing’s disease and the next day the dog returned for a low dose dexamethasone suppression test. A “pre” blood sample was collected and then 0.01 mg/kg dexamethasone was administered IV. Samples were collected at 4 and 8 hours post administration of dexamethasone. All three samples were centrifuged and serum was removed and placed into a separate, non-additive tube. The three samples were sent to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for cortisol measurement using the low dose dexamethasone suppression test. The results are presented in the Table 1.
After administration of low doses of dexamethasone, cortisol concentrations should decrease (suppress) for at least 8 hours. In general, the 8 hour result is used to characterize suppression as appropriate or inappropriate. In this case, the cortisol concentration did not stay decreased for 8 hours and suppression was considered inappropriate. In conjunction with the clinical signs and minimum database results, a diagnosis of Cushing’s disease was made.
The low dose dexamethasone suppression (LDDS) test is a fairly sensitive test. This means that false negatives are not common, though false negatives and especially false positives can still occur. While the LDDS test is a recommended screening test for most dogs suspected of having Cushing’s disease, it is important to rule out other diseases which can cause inappropriate suppression/false positives (i.e. diabetes, renal disease) with a minimum data base.
For more information about this case, contact Clinical Pathology Section Head Dr. Julie Piccione. To learn more about TVMDL’s test offerings, visit tvmdl.tamu.edu or call 1.888.646.5623.