Salivary cortisone to estimate cortisol exposure and sampling frequency required based on serum cortisol measurements

Context: Population studies frequently measure cortisol as a marker of stress and excess cortisol is associated with increased mortality. Cortisol has a circadian rhythm and frequent blood sampling is impractical to assess exposure. We investigated measuring salivary cortisone and examined sampling frequency required to determine cortisol exposure. Methods: Serum and saliva with cortisol and cortisone measured by LC-MS/MS in independent cohorts. The relationship between serum cortisol and salivary cortisone was analysed in cohort 1 using a linear mixed effects model and resulting fixed effects component was applied to cohort 2. Saliva cannot easily be collected when sleeping so we determined minimum sampling required to estimate cortisol exposure (eAUC) using 24-hour cortisol profiles (AUC24) and calculated the relative error (RE - a measure similar to the coefficient of variation) for the eAUC. Results: >90% of variability in salivary cortisone could be accounted for by change in serum cortisol. A single serum cortisol measurement was a poor estimate of AUC24 especially in the morning or last thing at night (RE > 68%), however 3 equally spaced samples gave a median RE of 0% (Interquartile range (IQR) between -15.6% and 15.1%). In patients with adrenal incidentalomas the eAUC based on 3 serum cortisol samples showed a difference between those with autonomous cortisol secretion and those without (p=0.03). Interpretation: Accepting that most people sleep 7-8 hours, using approximately 8-hourly salivary cortisone measurements provides a non-invasive method of estimating 24-hour cortisol exposure for population studies.