The potential for investigator-mediated contamination to occur during routine search activities

The sensitivity and discrimination power of modern DNA profiling systems means that very small amounts of DNA from an individual can be detected on an item leading to large inclusionary statistics for that person. The sensitivity of these systems has significant benefits in the investigation of crime but also can be highly sensitive to contamination of exhibits or crime scenes. It becomes critical to distinguish between deposition during commission of a crime or deposition via some other method unrelated to the crime. This study investigates methodologies used in crime scene examination and the potential for them to cause non-crime-related transfer of DNA. Factors assessed include the source of DNA, the handling time, the amount of movement during contact, and the substrate type. The amount of movement and the number of transfer steps are the most critical in determining whether, and how much, DNA is transferred. This study provides information for crime scene examiners and also scientists assessing transfer scenarios.