When forensic swabs are collected from an individual—whether from the skin or an intimate area, a measurable quantity of the person’s own DNA is generally expected. However, this expectation is not always met. In this study, we assessed the frequency of intimate swabs exhibiting unexpectedly low DNA quantities. Our aims were (i) to identify operational factors contributing to insufficient DNA recovery, and (ii) to highlight the importance for forensic laboratories to monitor endogenous DNA yield statistics to guide corrective actions when necessary.  

Approximately 850 intimate swabs (anal, vaginal, vulvar, urethral) collected from women between 2019 and 2025 were analyzed. Low female DNA concentrations (<0.1 ng/µL) were observed in 54% of anal margin swabs, 19% of vulvar swabs, and 4% of vaginal and urethral swabs. While the limited sampling area may partially explain the low yields observed from anal margin samples, the low quantities found in other types of swabs may indicate issues such as suboptimal sampling technique, mislabeling, or inadequate sample preservation. 

In French-speaking Switzerland, skin and intimate swabs are collected either by Forensic medical examiner or by gynecologists. Gynecologists, in particular, often have limited exposure to forensic practice and may not be familiar with crucial concepts such as chain of custody, contamination risks, or the specific utility and requirements of forensic sampling—many of which differ substantially from procedures used in clinical practice. To address these gaps, we established a training program delivered every six months, coinciding with the arrival of new Forensic medical examiner and gynecologists. This training covers evidence handling, labeling, contamination prevention, and appropriate forensic sampling techniques.  

In the future, comparing data collected before and after the implementation of the training program will provide valuable insight into its impact. At this stage, however, the number of post training samples remains too limited to allow a meaningful assessment. 

Authors

• Lydie Samie (Unit of Forensic Genetics - University Center of Legal Medicine, Lausanne - Geneva, Switzerland) 
• Castella Vincent (Unit of Forensic Genetics - University Center of Legal Medicine, Lausanne - Geneva, Switzerland)