Advancing methodological knowledge on hair cortisol analysis: prevention of storage effects, influence of scalp region in older adults and role of mean hair diameter/weight

• Applicant: Dr. Tobias Stalder
• Subject Area: Biological Psychology and Cognitive Neuroscience
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 503208201

Hair analysis for concentrations of cortisol (HCC) or other endogenous hormones is widely used in psychobiological research to assess long-term cumulative endocrine exposure. Still, key methodological influences remain insufficiently understood, threatening measurement validity and comparability across studies. Building on findings from our previous DFG project, this proposal addresses three critical unresolved issues: (1) Can adverse storage effects be prevented by freezing and vacuum-sealing of hair samples? We have shown that storage-related declines in HCC and other analytes already emerge after 6 months. This presents a major challenge for any research with a longer data collection phase, which is currently difficult to address. A method to prevent storage effects would be a major advancement. Study I is a first systematic investigation into whether frozen and vacuum-sealed storage of hair samples can prevent analyte degradation. In a within-subject study, temporal changes will be compared between frozen storage (–20°C, vacuum-sealed) vs. standard room temperature storage over four durations (0, 6, 12 or 24 months) using our established distributed sampling scheme. (2) Scalp regional difference: are findings transferable to older adults? Our previous work in young adults revealed an important influence of the scalp region from which hair is sampled on endocrine results. However, aging leads to significant changes in hair structure and growth behavior, meaning that scalp regional patterns are likely to be different in the critical group of older adults. Study II thus seeks to extend our knowledge on this vital influence using our previously established design, comparing hair sampling from the posterior vertex and the occipital region across two timepoints in older adults. (3) Does mean hair diameter/weight influence results? Our pilot data suggests that individual differences in the number of hairs per analytical sample (an index of mean hair diameter/weight) influences HCC results. However, this relationship requires validation in large samples, which is currently hindered by the lack of a time-efficient method for hair counting. In study IIIa, we thus plan to develop and validate an automated, machine learning-based method for hair counting, that can be integrated into routine laboratory protocols. In study IIIb, we then seek to apply this method in a large, heterogeneous dataset to allow for a comprehensive investigation of this relationship. In case of positive results, we will subsequently make the software freely available to enable widespread future monitoring of this covariate. The three studies of this project are likely to provide critical methodological insight and practical tools to improve data quality in future hair endocrine research. Findings will inform standardized protocols and minimize error variance, facilitating methodological reliability and comparability in future psychobiological and clinical research.

• DFG Programme: Research Grants
• Co-Investigators: Privatdozent Dr. Johannes Finke; Professor Dr. Tim Klucken; Professor Dr. Michael Möller