Protocols and challenges in managing multi-source biological evidence at the crime scene: a scoping review.

To maximise DNA yield from evidence and ensure the integrity of the chain of custody, all biological evidence must be properly managed and preserved during initial crime scene processing. This scoping review aimed to collate documented protocols and issues associated with processing blood, semen, saliva, and touch DNA at the crime scene to optimise downstream DNA processing. A synthesis review of 57 peer-reviewed studies (2015-2025) was conducted using a curated table of forensic DNA evidence. Data were presented in a table organised by study features, protocols, problems, solutions, results, and suggestions. Findings from the studies referenced favour optimised double-swab (wet + dry) or adhesive tape-lifting techniques with buffered solutions (e.g., PBS, SDS) and visualisation aids (e.g., Diamond Nucleic Acid Dye); immediate air-drying or active-drying systems; chemical preservatives (Longmire's buffer, ethanol, solid NaCl); cold storage; and carrier additives. Strict anti-contamination procedures (PPE, elimination databases, negative controls) are essential, as are separate tamper-evident packaging. These protocols improve DNA recovery, reduce degradation and stochastic effects, and enhance success in STR and mtDNA profiling. Key challenges include low-template stochasticity, shedder variability, substrate interference, environmental degradation (especially aquatic and thermal), operator variability, and secondary transfer. Most studies have been conducted in laboratories, which may not be generalisable to real-world settings. Rapid, substrate-tailored collection with minimal handling, immediate preservation, and a strict chain of custody constitute the current best practice for evidence collection at the crime scene. Future studies should focus on multi-site field validations, automation, and harmonised guidelines to address current translational gaps and enhance equity in forensic contexts.