The prevalence of mixed DNA profiles in fingernail samples taken from individuals in the general population

The fingernail hyponychium is an isolated area where biological material may accumulate and can provide a valuable source of evidential material in police investigations. DNA transfer between the victim and suspect frequently occurs during violent crimes and in court there is often reasonable doubt that a mixed DNA profile in a fingernail sample has originated from the assault as the profile may be attributed to previous contact between the two individuals. The purpose of this study was to assess background levels of foreign DNA under the fingernails of individuals from the general population in order to provide data that may help to determine whether DNA transfer occurred during or prior to the assault. Fingernail swabs sampled from 100 volunteers were processed by Qiagen™ extraction and amplified using AMPFlSTR® SGM Plus™ to obtain DNA profiles. Foreign DNA was detected in 13% of samples, with only 6% of these giving reportable mixed DNA profiles, suggesting the incidence of foreign DNA under the fingernails was low. A significant proportion of the mixed DNA profiles came from male donors; the majority had experienced physical contact within the 24 h time period prior to sampling.