Association between hair cortisol and diurnal basal cortisol levels: A 30-day validation study

This study aimed to validate the association between hair cortisol and 30-day integrated salivary cortisol levels. Additionally, the intra-individual stability of the hair cortisol and the association between hair cortisol and salivary cortisol in several time domains were systematically explored. Twenty-four adults (15 men and 9 women, mean age: 22.7 ± 2.8 years) were asked to collect three saliva samples per day for 30 days, immediately after awakening (time 1), 30 min after awakening (time 2), and at bedtime (time 3). The hair sample was taken from the posterior vertex as close as possible to the scalp at the end of the 30-day study period. The area under the curve based on times 1 and 3 showed that cortisol levels in the 1 cm hair segment closest to the scalp were moderately correlated with the 30-day salivary cortisol levels (r = 0.41, p = 0.047). However, this correlation was not observed for the cortisol awakening response, diurnal slope, or other metrics in different time domains. The mean salivary cortisol at time 3 was significantly correlated with hair cortisol (r = 0.42, p = 0.041). Intra-individual stability of the hair cortisol was maintained for at least 4 months. Hair cortisol may reflect basal cortisol levels more substantially over longer periods (longer than 1 month and for at least 4 month), thus reflecting longitudinal cortisol production, rather than the day-to-day temporal cortisol response. Salivary cortisol is an ideal metric for short-term or day-to-day stress response. Researchers should select suitable metrics, such as hair cortisol for basal level (or trait) or salivary cortisol for response (or state), depending on the study objective.