In vitro validation of vaginal sampling in rape victims: the problem of Locard’s principle

Abstract

Intra-vaginal traces can serve as supporting physical evidence of vaginal penetration in sex-crime cases. Vaginal trace sampling guidelines prescribe using a speculum and swabs or swabs only. The use of Locard's exchange theory requires samples of trace materials to always be contamination-free and of accurately known origin. If traces from the outer genital area are accidentally introduced into the vagina during sampling, these traces may be recorded as originating in a location where they were not present at the time of evidence collection. The goal of this study was to assess the risks of false positive findings in vitro of traces found inside the vagina due to trace dislocation caused by current vaginal trace sampling methods (swab-only or with speculum), and for a new method that uses a sleeve accessory to prevent trace dislocation. Starch solution was applied to the labia of three clean female genitalia mock-ups with vaginas of various diameters and structures. The speculum, swab-only, and sleeve methods were each used 30 times on each mock-up, giving a total of 90 repetitions of each method. Iodine was used to determine whether any starch traces had been introduced vaginally. The speculum and swab-only methods gave 100 and 63-87% false positive results respectively due to trace dislocation. The sleeve method reduced this to 2% (0% after correction). The results suggest that there is a need to conduct clinical in vivo studies to determine whether these results translate into everyday practice and to ensure reliable trace sampling in sex-crime cases.