Area under the curve and other summary indicators of repeated waking cortisol measurements

Objective: To derive the area under the curve and related summary measures of stress from saliva samples collected over time and to provide insight into the interpretation of the derived parameters. In research designed to assess the health consequences of stress these samples are often used as a physiologic indicator of the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis. To make these repeated measurements of salivary cortisol more useful in defining the relationships between stress and health there is a need to derive two forms of area under the curve that summarize the measurements: area Linder the curve with respect to ground (AUC(G)) and area under the curve with respect to increase (AUC(I)). The latter parameters, AUC(I) however, is seldom used by research scientists. Methods: In this study, interpretation and generic definition of the area under the curve was provided through graphical analyses and examination of its association with other summary measures using data from the Buffalo Cardio-Metabolic Occupational Police Stress (BCOPS) Pilot Study. In generic form, AUC(I) is derived as the area under the curve above the baseline value minus the area above the curve below the baseline value. Results: The sign and magnitude of AUC(I) are related to the profile and the rate of change of the measurements over time. The parameter showed significant associations with other summary indicators that measure pattern or rate of change of the measurements over time. Conclusion: Principal components analyses revealed that summary parameters derived from repeated cortisol measurements can be grouped into two meaningful general categories: measures of the magnitude of response and measures of the pattern of response over time.