title: Area under the curve and other summary indicators of repeated waking cortisol measurements.
abstract: 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 under 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Biostatistics and Epidemiology Branch, National Institute for Occupational Safety and Health, HELD/BEB, MS 4050, 1095 Willowdale Rd., Morgantown, WV 26505, USA. djf7@cdc.gov
authors.forename: Desta B
authors.initials: DB
authors.lastname: Fekedulegn
authors.forename: Michael E
authors.initials: ME
authors.lastname: Andrew
authors.forename: Cecil M
authors.initials: CM
authors.lastname: Burchfiel
authors.forename: John M
authors.initials: JM
authors.lastname: Violanti
authors.forename: Tara A
authors.initials: TA
authors.lastname: Hartley
authors.forename: Luenda E
authors.initials: LE
authors.lastname: Charles
authors.forename: Diane B
authors.initials: DB
authors.lastname: Miller
mesh_terms: D019540:Area Under Curve; D006801:Humans; D006854:Hydrocortisone / Q000032:analysis* / Q000493:pharmacokinetics; D025341:Principal Component Analysis; D012016:Reference Values; D015203:Reproducibility of Results; D012463:Saliva / Q000737:chemistry*; D014851:Wakefulness
chemical_list: D006854:Hydrocortisone