Evaluation of basal cortisol and the adrenocorticotropic hormone stimulation test in 982 dogs undergoing investigation for hypoadrenocorticism.

Basal cortisol screening has poor specificity, especially post-glucocorticoids; ACTH stimulation test provides added prognostic value.

This retrospective study evaluated the diagnostic performance of basal cortisol and the ACTH stimulation test (ACTH-ST) in 982 dogs investigated for hypoadrenocorticism, including 55 confirmed cases and 927 non-affected dogs. Dogs were stratified by prior glucocorticoid exposure (GS, n=198; NGS, n=729). Basal cortisol ≤2.0 μg/dL was significantly more prevalent in glucocorticoid-exposed dogs (46.0%) versus non-exposed dogs (31.0%), demonstrating that prior glucocorticoid therapy substantially increases false-positive screening results. At the standard ≤2.0 μg/dL cut-off, basal cortisol showed near-adequate sensitivity (98.2%) but poor specificity (65.8%), meaning many non-hypoadrenocorticoid dogs will screen positive. Raising the cut-off to 2.4, 2.7, or 3.1 μg/dL improved sensitivity marginally but further reduced specificity. False-positive flat-line ACTH-ST results occurred in 1.2% of dogs, highlighting that even the gold standard test has limitations. Importantly, ACTH-ST results provided prognostic value: in the NGS group, basal cortisol >7.0 μg/dL, post-ACTH cortisol >15.6 μg/dL, and delta cortisol <5.3 μg/dL were associated with increased risk of not surviving to discharge. These findings suggest that ACTH-ST not only confirms or excludes hypoadrenocorticism but also yields clinically meaningful prognostic data. Clinicians should interpret basal cortisol screening results cautiously in glucocorticoid-treated patients and recognize that higher cut-offs do not reliably improve overall diagnostic accuracy.

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