Biological Variability in Serum Cortisol Concentration Post‐adrenocorticotropic Hormone Stimulation in Healthy Dogs

BACKGROUND: The ACTH stimulation has low sensitivity for the diagnosis of hypercortisolism possibly as a result of biological and analytical variability. HYPOTHESIS/OBJECTIVES: To report the components of biological and analytical variability in serum cortisol concentration post‐ACTH stimulation ([cortisol]) in healthy dogs. ANIMALS: Fourteen healthy harrier hound dogs. METHODS: The data were extracted from a separate, prospective, randomized, double‐blinded, controlled discovery study in which dogs treated with vehicle control and 4 different doses of cortisone acetate (CA) for 7 days had an ACTH stimulation test performed to confirm the dose‐dependent effect of CA. The index of individuality (IoI), the critical difference between sequential measurements (C (D)), and the number of measurements required to assess the homeostatic set point (HSP) of [cortisol] with confidence intervals (CI) of 90 and 95% were estimated. RESULTS: The IoI was equal to 1.1 and the C (D) was 3.3 μg/dL (92 nmol/L). The number of measurements required to assess the HSP of [cortisol] with CI of 90 and 95% were 3 and 15, respectively. Additionally, mean [cortisol] was higher in males than in females (13.3 ± 4 μg/dL [366 ± 114 nmol/L] vs. 11.5 ± 2.5 μg/dL [318 ± 65 nmol/L], respectively; P = .046). As expected, treatment with CA resulted in a dose‐dependent suppression of [cortisol]. CONCLUSIONS AND CLINICAL IMPORTANCE: False‐negative test results in hypercortisolism could occur when [cortisol] is outside of the individual's HSP and within the reference interval. The large C (D) emphasizes the importance of assessing clinically relevant parameters in the diagnosis and monitoring of HC.