Biological variation of baseline serum cortisol concentration in healthy dogs, part 2: clinical applications

Introduction: Biological variation tools can be used to quantify uncertainty inherent in laboratory measurements and assist veterinarians in clinical decision making. Methods: Biological variation tools for interpretation of baseline cortisol concentrations in dogs are calculated using data obtained during part 1 of this study. Results: Expanded measurement uncertainty and bidirectional dispersion with 95% probability for baseline cortisol are 13.7 and 64.9% respectively, and these values can be used to establish a "grey-zone" around a population-based reference interval limit to guide clinical investigations. Index of individuality is intermediate (1.14), and forty-two samples would be needed to estimate an individual dog's homeostatic setpoint for cortisol. These values support current recommendations to interpret baseline cortisol concentrations in a broader clinical context utilizing ancillary testing when appropriate. The unidirectional reference change value for baseline cortisol is 77.3% with 95% probability, which can be helpful in determining the significance of changes in serial measurements. Discussion: Biological variation tools help define uncertainty around measurement of baseline cortisol concentrations in dogs, encourage an individualized approach to spectrum-of-care, and reinforce current consensus statements that rely on clinical signs combined with dynamic testing for the diagnosis of hyper- or hypoadrenocorticism rather than baseline cortisol concentrations alone.