"Shedding the status"—a meta-analysis on the individual propensity to shed skin material in the context of forensic trace analysis

The so-called "shedder status", which can more precisely be referred to as "individual shedding propensity" (ISP), has been the subject of forensic genetic research for more than two decades. Numerous studies have been published on this topic many of which report contradictory and/or insufficiently documented results regarding the existence, influencing factors, classifications of and test methodologies for the ISP of skin material. To date, there is no scientific consensus on the best way to register and conceptualize this variable, that is essential for the assessment of DNA transfer events. Here, we present the results of a systematic review and meta-analysis to give an account of the current state of research on the ISP (including data up to end of December 2023). To this end, 50 publications on the topic of "shedder status" were systematically surveyed and the combined data was statistically analysed regarding the comprehensibility and test methodology for determining the ISP as well as key influencing factors (including sex and age) and classifications. Despite difficulties in the analysis due to missing or incomplete information in individual publications, the following insights and recommendations are established, also considering current dermatological and histological research: the tendency to shed (mostly) skin material upon physical contact with surfaces is a genuine and meaningfully ascertainable, person-specific i.e. individual variable, which is apparently influenced by male sex and young age. In contrast, time since handwashing and the choice of hand do not appear to have any relevant influence. When testing an individual's tendency to deposit DNA containing material, it is preferable to set a standardised contact time and use plastic tubes as the contact surface or perform direct skin abrasion. The body region from which the sample is taken also appears to be relevant. Overall, however, the results confirm the difficulty to compare and interpret the results from many available studies, and we advocate to harmonise and standardise study designs and experiments and to apply a quality control before publishing such data.