Adrenocorticotropic hormone (ACTH) stimulation test

The adrenocorticotropic hormone (ACTH) stimulation test is the test most commonly used (but not necessarily the best) to confirm the diagnosis of hypercortisolism in dogs. It is a simple test that is relatively inexpensive and quick. By evaluating the response of the adrenal glands to the administration of exogenous ACTH, it is possible to support the diagnosis of hypercortisolism. In addition, this test makes it possible to identify subjects with iatrogenic hypercortisolism.

Performing the test

Numerous protocols have been published in the literature; however, the one most commonly used is performed using the following procedure:

1. Blood sampling (serum or EDTA plasma) for measurement of the basal blood cortisol level;
2. Intravenous or intramuscular administration of 0.25 mg (0.125 mg in dogs weighing <5kg) of synthetic ACTH (tetracosactide hexacetate). Studies performed in countries in which the drug is extremely expensive have shown that a dose of 5 μg/kg is sufficient to obtain maximum adrenal stimulation;
3. After 60 minutes, a second blood sample is taken for the assay of the post-stimulation blood cortisol level;
4. The two samples are sent to the laboratory; since cortisol is a quite stable hormone, it is sufficient to send the samples chilled and not necessarily frozen.

Interpretation of results

A normal subject has basal blood cortisol values between 0.5 and 6.0 μg/dl. Post-stimulation values are typically between 6 and 17 μg/dl. Post-stimulation values between 17 and 22 μg/dl are considered equivocal and non-diagnostic, while post-stimulation values higher than 22 μg/dl are indicative of hypercortisolism.

Subjects with iatrogenic hypercortisolism have low or normal basal blood cortisol values; however, these values do not differ following the administration of exogenous ACTH.

This test makes it possible to confirm hypercortisolism in 80-85% of dogs with pituitary-dependent hyperadrenocorticism and in 50-60% of dogs with adrenal-dependent hyperadrenocorticism (ADH). The high percentages of false negative test results associated with ADH derive from the lack of sensitivity to ACTH that sometimes occurs in the course of adrenal neoplasms.