Individual DNA shedding propensity: scoping review and proposed framework

The individual variation in depositing DNA traces, historically termed "shedder status," represents a critical variable in forensic activity level evaluations. However, the integration of this concept into casework is currently hindered by inconsistent terminology, a lack of standardized definitions, and disparate experimental methodologies. This paper presents a conceptual framework for Individual Shedding Propensity (ISP) and a scoping review of 130 relevant publications to synthesize current knowledge and identify gaps. We propose defining ISP as a stable, intrinsic personal characteristic that governs the accumulation and transfer of endogenous DNA-containing biological material. We distinguish between intrinsic propensity and transient environmental or behavioural factors. Within this framework, shedding is dissected into accumulation and transfer phases. The scoping review confirms that inter- and intrapersonal variations in shedding are indisputable. However, the ability to draw overarching conclusions is limited by widespread variation in testing methods. Classification systems vary significantly between studies, rendering much of the existing data difficult to compare. The review further analyses influencing factors, noting that while some biological traits (e.g., skin conditions) and behaviours (e.g., handwashing) are well-documented, environmental factors that interact with ISP require further investigation. To advance the field, we recommend moving toward a quantifiable and standardized ISP measure. We advocate for the harmonization of research through a standardized ISP-test-specifically utilizing the plastic tube method-to generate reproducible "ISP-scores". Furthermore, the establishment of a centralized database for sharing ISP data is proposed.