ClinQuiz: Interpreting low-dose dexamethasone suppression test results

Test your knowledge of what these LDDS test results mean with this short quiz. But don't worry, unlike the exams you took in school, the answers, with complete explanations, are provided.
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Feb 01, 2009
ClinQuiz
Bruyette
David S. Bruyette, DVM, DACVIM

The low-dose dexamethasone suppression (LDDS) test is commonly used as a screening test for hyperadrenocorticism in dogs, and, in some cases, the results can be used to distinguish pituitary-dependent hyperadrenocorticism (PDH) from adrenal-dependent hyperadrenocorticism (ADH), or an adrenal tumor.

The LDDS test is typically performed in dogs that have fasted for eight hours (check whether your laboratory requires fasted samples). To perform this test, obtain a baseline (0 hour) blood sample, administer dexamethasone sodium phosphate (0.015 mg/kg) or aqueous dexamethasone (0.01 mg/kg) intravenously, and obtain two additional blood samples four and eight hours later. Contact your laboratory before testing to determine its specific LDDS test protocol. The serum cortisol concentrations are measured in the three blood samples.

When interpreting LDDS test results, first evaluate the eight-hour postdexamethasone administration cortisol concentration. If it is above the reference range, the dog probably has hyperadrenocorticism (false positive results may occur in dogs with nonadrenal illness). If it is within the reference range, either the dog does not have hyperadrenocorticism or there is a 5% to 10% chance that the dog has PDH. (The dog may have early pituitary disease and the pituitary gland is still responding to a pharmacologic dose of dexamethasone by decreasing ACTH production, thereby reducing serum cortisol concentrations.) In those cases, an ACTH stimulation test is warranted.

If the eight-hour postdexamethasone administration cortisol concentration is above the reference range, then evaluate the baseline and four-hour postdexamethasone administration cortisol concentrations to see whether cortisol suppression occurred during the eight hours. If at least 50% cortisol concentration suppression is present at the four- or eight-hour time points, the definitive diagnosis is PDH and additional adrenal function tests are not needed.

The four case examples in this quiz help illustrate some of the common pitfalls in LDDS test result interpretation and questions practitioners have after receiving the test results. For the purposes of this quiz, the reference range for the eight-hour postdexamethasone administration cortisol concentration is ≤ 2 g/dl; however, this cutoff value varies by laboratory.

Q1

1. Which statement applies to these LDDS test results (more than one answer may apply)?

A. The dog has hyperadrenocorticism.
B. The dog has hyperadrenocorticism secondary to an adrenal tumor (ADH).
C. The dog has PDH.
D. PDH and ADH cannot be differentiated based on these results.
E. None of the above

Q2

2. Which statement applies to these LDDS test results (more than one answer may apply)?

A. The dog has hyperadrenocorticism.
B. The dog has hyperadrenocorticism secondary to an adrenal tumor (ADH).
C. The dog has PDH.
D. PDH and ADH cannot be differentiated based on these results.
E. None of the above

Q3

3. Which statement applies to these LDDS test results (more than one answer may apply)?

A. The dog has hyperadrenocorticism.
B. The dog has hyperadrenocorticism secondary to an adrenal tumor (ADH).
C. The dog has PDH.
D. PDH and ADH cannot be differentiated based on these results.
E. None of the above

Q4

4. Which statement applies to these LDDS test results (more than one answer may apply)?

A. The dog has hyperadrenocorticism.
B. The dog has hyperadrenocorticism secondary to an adrenal tumor (ADH).
C. The dog has PDH.
D. PDH and ADH cannot be differentiated based on these results.
E. None of the above

This quiz was provided by David S. Bruyette, DVM, DACVIM, VCA West Los Angeles Animal Hospital, 1818 S. Sepulveda Blvd, West Los Angeles, CA 90025.