Assessing the presence of female DNA on post-coital penile swabs: Relevance to the investigation of sexual assault

During the investigation of sexual assault, penile swabs from an alleged perpetrator are processed as part of the routine procedure. The results of the forensic DNA analysis may subsequently be presented in court. Documentation on the expectancy of finding female cells on post-coital penile swabs is scarce. Reviews from assault cases show that retrieval of female cells from penile swabs is generally reported for less than 50% of the cases. The perception may therefore be that female cells are not expected to be recovered on penile swabs, even few hours after vaginal penetration. However, these reviews do not provide certainty that penetration of body cavities had taken place. Furthermore, the alleged perpetrator may have washed himself before the examination. In the present study, eleven couples provided fourteen sets of post-coital penile swabs collected between 5 and 24 h after vaginal intercourse. At time intervals between 5 and 12 h, the full female DNA profiles were recovered in 90% of the samples. At the lowest, 67% of the full female profile was typed as an average of two swabs sampled at each time point. Samples collected from three couples at 20, 22 and 24 h post-coital, retrieved 100% (15 AMPFlSTR®Identifiler markers, in addition to the amelogenin gene) of the female DNA profile from one couple, and 37% and 30% of the full female DNA profiles from the other two couples. For the majority of samples, male DNA was present in slightly greater abundance than female DNA. In this study, female DNA was recovered on all post-coital penile swabs taken at 5–24 h intervals.