DNA contamination represents one of the most frequent sources of error in forensic genetics and can have major consequences for the judicial system. Monitoring its occurrence and detecting incidents as early as possible is essential for ensuring the reliability and integrity of forensic caseworks. Previous studies estimate that approximately 0.1–1% of profiles submitted to forensic databases correspond to contamination. However, these figures likely underestimate the true amount of contamination, particularly in complex mixtures. While contamination screening in single-source profiles is relatively straightforward through routine comparisons with staff DNA databases, mixed profiles are rarely subjected to systematic checks. This largely reflects the limitations of CODIS™, the software used in most countries for database searches, which is not designed for comparisons involving complex mixtures. However, with the advent of probabilistic genotyping, such comparisons can now be conducted efficiently, enabling the routine detection of contamination even in highly complex DNA profiles.

In this study, we examined all the mixture profiles from a single police service that have been deconvoluted using STRmix™ between 2019 and 2025. Each deconvoluted mixture was compared against staff reference profiles from both the police service and the laboratory using the STRmix™ database search module. The proportion of contamination highlighted in mixtures was then compared to contamination rates detected through standard CODIS staff index searches conducted during the same period for the same police service. This retrospective analysis provides a first estimate of staff contamination rates in complex mixture profiles. It further demonstrates the added value of incorporating staff contamination screening into mixture analysis workflows, highlighting its potential to strengthen data integrity, improve detection of contamination events, and enhance the overall robustness of forensic DNA casework.

Authors

• Patrick Basset (Forensic Genetics Unit, University Center of Legal Medicine Lausanne-Geneva, Lausanne University Hospital and University of Lausanne, Switzerland)
• Louanne Toulemont (Forensic Genetics Unit, University Center of Legal Medicine Lausanne-Geneva, Lausanne University Hospital and University of Lausanne, Switzerland)
• Vincent Castella (Forensic Genetics Unit, University Center of Legal Medicine Lausanne-Geneva, Lausanne University Hospital and University of Lausanne, Switzerland)