Prevalence and persistence of foreign DNA beneath fingernails

Fingernail scrapings and clippings are routinely examined for the presence of foreign DNA profiles in forensic casework where the case history suggests their evidentiary relevance. In order to better understand the significance of these findings, casework results from the Centre of Forensic Sciences (CFS) were analyzed and several controlled studies were conducted. In an analysis of casework data (n=265), 33% of fingernail samples contained a foreign source of DNA, 63% of which were detected at 5 or more STR loci. In a sampling of fingernails from the general population (n=178), 19% contained a foreign source of DNA, 35% of which were detected at 5 or more STR loci. In a study involving deliberate scratching of another individual (n=30), 33% of individuals had a foreign DNA profile beneath their fingernails from which the person they scratched could not be excluded as the source; however when sampling occurred ≈ 6 h after the scratching event, only 7% retained the foreign DNA. This research suggests the incidence of foreign DNA profiles beneath fingernails in the general population is low but, when present, the majority is of limited significance and tends not to persist for an extensive period of time. These data are provided to assist the forensic analyst when providing his or her opinion as to the relevance of foreign DNA present under fingernails.