Saliva cortisol profiles in field research: internal structure, confounding factors, quantification, and stability

The last decades of research on hypothalamus-pituitary-adrenal axis activity have established salivary cortisol as a prevalent biological marker of stress. Diurnal salivary cortisol profiles in healthy adults are characterized by peak values in the post-awakening period and a consecutive decline thereafter. Basically, the variety of published measures quantifying basal salivary cortisol profiles can be classified into level and dynamic parameters reflecting both state and trait aspects. A total of 440 participants working in the human service sector were investigated for diurnal salivary cortisol levels in their natural environment. The aim of the present study was to analyse the internal structure of the cortisol data and to assess the influence of potential confounding factors, to which salivary cortisol levels are reported to be sensitive. In addition, the stability of cortisol profiles, single samples and composite measures was examined to determine whether the aggregation of cortisol values and their appropriateness as trait measures of HPA-axis functioning can be empirically justified. All in all, only few confounding factors proved to account for significant differences in cortisol levels. Average diurnal salivary cortisol profiles proved to be very stable and reflected the expected circadian cortisol rhythm with higher cortisol levels in the post-awakening period and a subsequent significant decline in cortisol for the remainder of the day. Parameters reflecting the average level of cortisol output proved to be of moderate to high stability, while parameters reflecting dynamic changes revealed moderate to poor stability. The large variability in cortisol values resulting in moderate stability coefficients is of concern regarding the diagnostic appropriateness of basal cortisol levels in field studies in contrast to experimental studies. Longitudinal study designs are needed to determine how short-term and long-term stress-related consequences are reflected in basal salivary cortisol levels