Basal or Resting Cortisol Concentration

• Convenient and inexpensive way to rule out hypoadrenocorticism. Recommend in dogs that have nonspecific signs (inappetence, weight loss, vomiting, diarrhea) that could be due to glucocorticoid deficiency without electrolyte abnormalities.
• >2 mg/dL (55 nmol/L)—Rules out hypoadrenocorticism.
• 1–2 mg/dL (28–55 nmol/L)—Hypoadrenocorticism is less likely, but an ACTH stimulation test is needed to confirm/ rule out the diagnosis.
• <1 mg/dL (28 nmol/L)—An ACTH stimulation test is needed to confirm/rule out the diagnosis.
• Many laboratories have decreased the detection limit of the cortisol assay from 1 mg/dL (28 nmol/L) to 0.2 mg/ dL (5.5 nmol/L). Although this has improved the ability of the basal cortisol concentration to differentiate dogs with and without hypoadrenocorticism (i.e., a cortisol concentration <0.2 mg/dL in a dog that has not received corticosteroids is highly suspicious for hypoadrenocorticism), an ACTH stimulation test or cortisol to ACTH ratio is still recommended to confirm/rule out the diagnosis in dogs with a low basal cortisol concentration.

ACTH Stimulation Test

• Still considered the gold standard for diagnosing hypoadrenocorticism.
• Recommend in dogs that have clinical signs consistent with hypoadrenocorticism and hyponatremia and/or hyperkalemia.
• Can use a 1 μg/kg dose of synthetic ACTH (cosyntropin or tetracosactrin) for screening. Repeat with a 5 μg/ kg dose to confirm the diagnosis if results are borderline/equivocal with the 1 μg/kg dose.

Cortisol to ACTH Ratio (CAR)

• Single blood sample can be used to confirm/rule out primary hypoadrenocorticism.
• Eliminates the need for synthetic ACTH and to perform an ACTH stimulation test.
• Samples must be collected before the administration of corticosteroids.
• Sample handling important for endogenous ACTH measurement (review laboratory recommendations)—Immediately centrifuge, separate plasma from cells, and freeze until shipping. Ship overnight on ice. This will minimize the degradation of ACTH, which is a concern with inappropriate sample handling.
• Unlike the basal cortisol and endogenous ACTH concentrations, there is no overlap of the CAR between healthy dogs, dogs with non-adrenal illness, and dogs with hypoadrenocorticism.
• Not useful for cases of secondary hypoadrenocorticism (low ACTH). An ACTH stimulation test is recommended to rule out/confirm hypoadrenocorticism in dogs with low basal cortisol and ACTH concentrations.

Interpretation

• Calculate the ratio of basal cortisol to ACTH.
• CAR=cortisol (mg/dL)¸ ACTH (pg/mL)
• Median CAR from Lathan et al. J Vet Intern Med. 2014
• Healthy=2.27
• Non-adrenal illness=2.84
• Hypoadrenocorticism=0.000714
• CAR range from Lathan et al. J Vet Intern Med. 2014
• Glucocorticoid deficient hypoadrenocorticism: 0.0004–0.01
• Mineralocorticoid and glucocorticoid deficient hypoadrenocorticism: 0.0004–0.0310

References

1.  Lathan P. Hypoadrenocorticism in dogs. In: J Rand (Ed.), Clinical Endocrinology of Companion Animals. Ames, Iowa: John Wiley & Sons, Inc; 2013.

2.  Scott-Moncrieff JC. Hypoadrenocorticism. In: EC Feldman, RW Nelson, C Reusch, JC Scott-Moncrieff (Eds). Canine and Feline Endocrinology, Edition 4. Elsevier Health Sciences; 2015.

3.  Lennon EM, Boyle TE, Hutchins RG, Friedenthal A, Correa MT, Bissett SA, Moses LS, Papich MG, Birkenheuer AJ. Use of basal serum or plasma cortisol concentrations to rule out a diagnosis of hypoadrenocorticism in dogs: 123 cases (2000–2005). JAVMA. 2007;231(3):413–416.

4.  Gold AJ, Langlois DK, Refsal KR. Evaluation of basal serum or plasma cortisol concentrations for the diagnosis of hypoadrenocorticism in dogs. J Vet Intern Med. 2016;30:1798–1805.

5.  Lathan P, Scott-Moncrieff JC, Wills RW. Use of the cortisol-to-ACTH ratio for diagnosis of primary hypoadrenocorticism in dogs. J Vet Intern Med. 2014;28:1546–1550.