Basal Serum Cortisol Concentration as a Screening Test for Hypoadrenocorticism in Dogs

BACKGROUND: Measurement of basal serum or plasma cortisol concentration is used as a screening test for hypoadrenocorticism in dogs, but is not well characterized. OBJECTIVES: To evaluate the sensitivity and specificity of basal serum cortisol to detect hypoadrenocorticism in a population of dogs with a clinical suspicion of hypoadrenocorticism. ANIMALS: Four hundred and fifty dogs with nonadrenal gland illness and 14 dogs with naturally occurring hypoadrenocorticism were included. METHODS: Retrospective case‐control study. The records of all dogs having had an ACTH stimulation test performed between January 2005 and September 2011 at the University of Bristol were reviewed. Dogs were included if the test was performed as a screening for hypoadrenocorticism. The sensitivity and specificity of basal serum cortisol concentration to detect dogs with hypoadrenocorticism were calculated using 2 cut‐offs and compared to the gold standard ACTH stimulation test. RESULTS: Using a cut‐off of ≤2 μg/dL (≤55 nmol/L), the sensitivity and specificity of basal cortisol to detect hypoadrenocorticism were 100% and 63.3%, respectively, whereas for a cut‐off of ≤1 μg/dL (≤28 nmol/L), the sensitivity and specificity were 85.7% and 91.8%, respectively. CONCLUSIONS AND CLINICAL IMPORTANCE: Measurement of basal serum cortisol is useful as a screening test for hypoadrenocorticism in dogs using a cut‐off of ≤2 μg/dL (≤55 nmol/L), and the disease is unlikely with a basal serum cortisol >2 μg/dL (>55 nmol/L). A basal serum cortisol ≤2 μg/dL (≤55 nmol/L) cannot be used to diagnose hypoadrenocorticism, and an ACTH stimulation test should be performed in these cases.