LABORATORY TESTING FOR ADRENAL DISEASE

38 LABORATORY TESTING FOR ADRENAL DISEASE



1 What causes canine hyperadrenocorticism?


Hyperadrenocorticism (HAC) in dogs is a condition in which serum cortisol levels are persistently elevated due to either an adrenal tumor that secretes cortisol or a pituitary tumor that secretes adrenocorticotropic hormone (ACTH, adrenocorticotropin), thus continuously stimulating release of cortisol from the adrenal gland. Pituitary-dependent hyperadrenocorticism (PDH) accounts for about 85% of canine HAC cases and adrenal-dependent hyperadrenocorticism (ADH) for about 15%. It is important to determine which type is present in dogs with HAC because therapy options differ.



2 What is the best test to screen for canine hyperadrenocorticism?


No consensus exists regarding the best test to screen for canine HAC. The tests used most often are the low-dose dexamethasone suppression test (LDDST) and the ACTH stimulation test. Other tests used less often are the urine cortisol/creatinine ratio and the combination LDDST/ACTH stimulation test. None of these tests has both extremely high sensitivity (ability to detect disease when present) and specificity (ability to rule out disease when absent). When dogs with typical clinical signs and other laboratory findings consistent with HAC are tested with one of the screening tests, the results are often conclusive, but in some cases, more than one assay is required or other diagnostic modalities are needed.



3 How is the low-dose dexamethasone suppression test performed and interpreted in dogs?


The LDDST is best started between 8 and 9 AM. A baseline blood sample for cortisol assay is taken, then 0.01 mg/kg body weight of dexamethasone or dexamethasone sodium phosphate is given intravenously (IV). Blood samples for cortisol assay are taken 3 to 4 hours and 8 hours after dexamethasone injection. Cortisol levels in normal dogs are decreased to less than 1.0 μg/dl at both 4 and 8 hours. Dogs with HAC generally have cortisol concentrations greater than 1.4 μg/dl at 8 hours, with cortisol levels of 1.0 to 1.4 μg/dl considered nondiagnostic.


The cortisol concentration 4 hours after dexamethasone injection may help distinguish between PDH and ADH in some dogs. If the cortisol level is suppressed (defined as <1.4 μg/dl or <50% of baseline concentration) at 4 hours, the dog most likely has PDH because dogs with ADH will not be suppressed at 4 hours. About 40% of PDH dogs also will not be suppressed at 4 hours, so lack of suppression at 4 hours is not diagnostic.



4 What are the advantages and disadvantages of the LDDST?


The low-dose dexamethasone suppression test probably has a higher sensitivity (ability to detect presence of HAC) and lower specificity (ability to rule out HAC) than the ACTH stimulation test. LDDST is relatively easy to perform and interpret and may distinguish between PDH and ADH, thus eliminating the need for further testing. LDDST cannot be used to diagnose iatrogenic HAC. The test is affected by stress, non-HAC illnesses, and drugs such as phenobarbital. The LDDST takes 8 hours to complete.



5 How is the ACTH stimulation test performed and interpreted?


A baseline blood sample is collected for cortisol assay, then 2.2 units/kg of aqueous ACTH gel is given intramuscularly (IM), or 250 μg of synthetic ACTH (cosyntropin) is given IM or IV. Lower doses of synthetic ACTH have also been used successfully for this test, but a single vial contains 250 μg, so this amount is most often used regardless of the dog’s size. A second blood sample for cortisol assay is collected 2 hours after ACTH injection, if the gel is used, or 1 hour after injection if synthetic ACTH is used. Interpretation of results may vary between laboratories. In general, normal dogs will stimulate to a cortisol level of 6 to 18 μg/dl. Stimulation to18 μg/dl or greater may be interpreted as being consistent with HAC, depending on the reference ranges and interpretation provided by the laboratory performing the assay.



6 What are the advantages and disadvantages of the ACTH stimulation test?


The ACTH stimulation test appears to have a somewhat lower sensitivity but higher specificity for diagnosis of canine HAC than does the LDDST; that is, a relatively high percentage of dogs with HAC, especially those with ADH, do not stimulate to higher than reference ranges. However, the ACTH test reportedly is better at ruling out HAC when it is not present, especially when nonadrenal illness causes a false-positive LDDST response. The ACTH stimulation test is the only test used to detect iatrogenic HAC and to monitor medical therapy for HAC. It does not distinguish between ADH and PDH. The amount of time necessary to do the ACTH stimulation test is less than that needed for the LDDST, and the ACTH stimulation test may be performed at any time of the day.



7 Is the combined LDDST/ACTH stimulation test a good way to test for and differentiate between types of canine hyperadrenocorticism?


Although it seems a convenient way to combine screening for HAC and testing for ADH versus PDH, the combined LDDST/ACTH stimulation test does not appear to be as accurate for either purpose than other available tests and is generally not recommended.



8 How useful is the urine cortisol/creatinine ratio in screening for canine hyperadrenocorticism?


Urinary cortisol excretion is increased in HAC, but cortisol concentration in urine varies with the amount of urine produced at any given time. Urine creatinine is excreted at a constant rate throughout the day, so measurement of creatinine concentration in the same sample provides a method of canceling out the effects of urine volume. The sensitivity of this test appears to be very high, that is, the ratio will be elevated in almost all dogs with HAC. Because many non-HAC conditions also result in an elevated urine cortisol/creatinine ratio, the test has very low specificity and has value only in ruling out HAC.

Related posts:

  1. ERYTHROCYTE DISORDERS
  2. OVERVIEW: HEMOSTATIC COMPONENTS AND DISORDERS
  3. INTERPRETATION OF THE LEUKOGRAM
  4. METABOLIC ACID-BASE ABNORMALITIES

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on LABORATORY TESTING FOR ADRENAL DISEASE

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