The transfer and persistence of DNA under the fingernails following digital penetration of the vagina

Acts of aggression and defence during sexual assaults often lead to transfer of DNA from victim to assailant. Of sexual assault cases recorded by the FSS between 2005 and 2010, approximately 20% and 3% involved digital penetration of the victim's vagina or anus, respectively. Of the 309 cases where digital penetration was alleged, a suspect was arrested within approximately 1 day in 135 cases and fingernail samples were taken in 101 of these cases. In this study the transfer and persistence of DNA under the fingernails after an act of digital penetration were examined, (a) to assist investigators in determining whether to obtain such samples initially and (b) to assist evaluators in assessing the evidential value of female DNA profiles post analysis. Full female profiles were obtained from all swabs collected at 0 and 6 h after digital penetration, indicating that female DNA was always transferred and persisted in the short term. Furthermore, full female profiles were produced from three-quarters of samples collected after 12 h whilst mixed profiles were produced in the majority of samples taken after 18 h. The analysis of several variables indicated that hand washing had a significant effect on the persistence of female DNA profiles.