Comparison of DNA profiles from samples collected from underneath fingernails and hand deposits following everyday activity

In instances of direct physical contact between individuals involved in criminal activity, body samples can provide significant and relevant information to aid in criminal investigations and court proceedings. Fingernails are one such forensically relevant body area that is capable of providing evidence of direct contact and potentially revealing whether the interaction involved was of a forceful kind. Several studies have investigated the prevalence of non-self DNA under fingernails after different crime-related scenarios; however, few have assessed the types of DNA profiles found after everyday activities. Furthermore, the comparability of the fingernail samples to those deposited on contacted surfaces remains unknown. In this study, we examined the composition of self- and non-self-DNA in samples collected from under the fingernails and held tubes from the same set of individuals. Additionally, the potential use of fingernails samples for shedder assessment was evaluated through comparison with two common shedder categorisation tests. For these purposes, samples were collected from both hands of 25 individuals of different demographics, without any prior restrictions on activities. Direct deposits were made by holding a 50 mL tube (for DNA shedder testing) and placing index fingers onto a slide (for Diamond™ dye cell counting shedder testing). Fingernail samples from both hands were taken immediately after tube-holding deposits. Reference DNA samples were collected from the participants as well as their cohabitating partners and other adults. Qualitative and quantitative data on DNA and cell deposits were collected to support activity-level evaluations. In our study, mixture inversions were rare, with non-self DNA, when detected, usually present as a minor component. More non-self DNA was detected after participants’ contact with the tube compared to fingernail samples. Partners’ DNA was frequently detected in both sample types, but more so in fingernail samples. Comparisons of the three shedder testing methods (fingernails, tube holding and cell count) showed that the categorisation results of these methods are not interchangeable and that DNA methods (tube vs. fingernails) were more consistent (64 % of deposited classified into the same shedder category) with each other than with cell counts (tube vs. cell count: 52 % classified into the same shedder category) (fingernails vs. cell count: 40 % remained in the same category). We anticipate that these datasets will serve as a valuable resource for activity-level evaluations and encourage other investigators to contribute to the growing data collection.