Hair cortisol measurement: A systematic review of current practices and a proposed checklist for reporting standards

Summarising hair cortisol concentration (HCC) methodology may provide much-needed data toward protocol standardisation to maximise future comparability of findings across studies. We searched five electronic databases, reviewing 11,716 publications focused on protocols previously used to measure hair cortisol. Our aim was to determine the frequency with which each procedure was reported in the literature. We then conducted a meta-analysis of the HCC results and proposed a checklist for reporting methodological procedures related to HCC. Using pre-selected key terms, we searched for population-based, non-experimental studies reporting HCC outcomes published up to November 2023. Eighty-seven analytical samples were included in the qualitative analysis and 28 in the quantitative analysis. The analyzed studies predominantly included children (≤10 years; 45.4 %) and mainly involved participants from European populations (72.6 %). There was significant variation in hair sample collection procedures across the studies. Most used hair samples up to 3 cm in length (92 %), with around one-third employing either milled (33.3 %) or minced (29.9 %) as grinding methods. For quantification, LC-MS was the most common method (47.1 %), followed by ELISA (24.1 %). Meta-analysis showed significant variability in the mean HCC observed. Meta-regression showed no association between differences in methodology and HCC. In conclusion, the absence of a standardized protocol in HCC research may result in procedural variability, making it difficult to compare findings across studies. Many published studies lacked sufficient detail in describing their methods. To address this, we propose a checklist of reporting guidelines for measurement procedures related to HCC.