Exploring Reference Values for Hair Cortisol: Hair Weight versus Hair Protein.

BACKGROUND As a marker of cumulative cortisol activity, hair cortisol has received attention in clinical and methodological research. Currently, it is common practice to relate hair cortisol concentration (HCC) to hair weight. The present paper explores hair protein concentration (HPC) as another possible reference value for HCC. METHODS For n = 18 hair samples cut from the posterior vertex, the HCC, HPC, and hair sample weight were determined, and the cortisol-to-weight and cortisol-to-protein ratios were calculated. Correlations were analyzed between HCC, HPC, and hair sample weight as well as between the cortisol-to-weight and cortisol-to-protein ratios. Hair sample weight and HPC were included as independent variables in a stepwise linear regression model to predict HCC. RESULTS HCC and HPC did not correlate significantly (r = .393, p = .106); however, the correlation between HCC and hair sample weight was significant (r = .520, p = .027). HPC and hair sample weight (r = .605, p = .008) as well as the cortisol-to-weight and cortisol-to-protein ratios (r = .858, p < .000) showed a high correlation. Hair sample weight was the better predictor of HCC (β = .520, p = .027) than HPC (β = .125, p = .657). CONCLUSIONS The results indicate that hair sample weight is the more suitable reference value for HCC. Thus, the standard cortisol-to-weight ratio should be used as the preferred expression for cumulative cortisol activity measured in scalp hair. However, calculating the cortisol-to-protein ratio can be considered as an alternative if the hair sample weight is not available.