ENDOCRINOLOGY
Disadvantage
• It does not distinguish between PDH and ADH
• It is not very sensitive in detecting HAC caused by
adrenal neoplasia
Cortisol concentration (nmol/l)
250
C. Dexamethasone suppression tests:
Low dose Dexamethasone suppression test (LDDST)
The low dose dexamethasone suppression test is an alternative to the ACTH stimulation test for the diagnosis
of HAC.
Method:
• Collect a blood sample for the basal cortisol concentration
• Inject 0.01 mg/kg dexamethasone iv
• Collect samples at 4 (-6) and 8 hours post-dexamethasone injection.
Interpretation (Figure 1)
• A normal response is suppression to below 40 nmol/l
• A complete lack of response could be consistent
with PDH or ADH
• In most PDH cases, there will be suppression to below 40 nmol/l at 4-6 hours and “escape” at 8 hours
with an increase in cortisol concentrations.
Advantages
• The sensitivity is higher than the ACTH stimulation
test (90-95%), thus fewer cases will be missed.
• It has an almost 100% sensitive with adrenal neoplasia and 90 -95% sensitivity for PDH
Disadvantages
• The specificity however is very low if measured in
sick dogs ( 44%- 73%) - thus a ≅ 50% chance of false
positives in non-adrenal illness. 5
• The test takes a long time (8h)
• It does not distinguish between iatrogenic and spontaneous HAC.
A disadvantage of both the ACTH stimulation test
and the LDDST is that false positives may occur in
animals with other chronic diseases. These include
renal and hepatic disease, diabetes mellitus, neoplasia and chronic inflammation.
False positives can be reduced by selecting patients
carefully. Typical clinical signs of HAC should be present in patients selected for these tests and concurrent disease should be ruled out by clinical examination, clinical pathology and imaging.
High Dose Dexamethasone Suppression Test
This is NOT a screening test. It is used for discriminating
between pituitary-dependent and adrenal-dependent hyperadrenocorticism. It has become less frequently used
since ultrasound is available in most practices.
Method:1,3
• Collect a blood sample for the basal cortisol concentration
• Inject 0.1 mg/kg dexamethasone iv
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Issue 06 | DECEMBER 2015 | 12
200
150
Normal
PDH of ADH
50
0
Figure 1
•
PDH
100
0
4
2
6
8
Time (hours)
Collect samples at 6 and 8 hours post-dexamethasone.
Interpretation:
• No suppression is consistent with ADH. The neoplastic cells function autonomously and are not
subject to feedback suppression.
• Suppression to below 40 nmol/l is consistent with
PDH.
D. Endogenous ACTH
This is the preferred test for HAC in horses but has not
been used extensively in dogs.
Method:
The sampling conditions are very specific.
• The blood should be taken into chilled EDTA-aprotinin tubes, placed immediately on iced and separated immediatly refrigerated centrifuge.4
• The plasma should be transferred into a PLASTIC
tube and immediately frozen and sent to the laboratory frozen.
Interpretation3
• Normal dogs have an ACTH concentration of 3-10
pmol/l
• Dogs w ]]