During a physical assault, biological material can be transferred between an assailant and victim. When victims defend themselves with their hands, biological material from the assailant may accumulate underneath their fingernails. Currently, research on fingernail evidence is limited; there is no optimized method regarding fingernail evidence processing. It is unknown what type of packaging best prevents loss and degradation of DNA during transport, and what procedures are best for recovering the maximum amount of exogenous DNA while limiting the amount of nail DNA. In this research, exogenous DNA recovery was optimized. Factors investigated included: comparison of soaking solutions; length of soaking time; length of digestion; vortexing; the effect of pelleting exogenous material for extraction; and the comparison of improved organic and silica-based extractions. Female volunteers then scratched male volunteers, and the improved procedure was applied. Next, different kinds of packaging were compared, noting if DNA degradation and loss occurred. DNAs were quantified and autosomal and Y-STRs were analyzed. Soaking the fingernail for 15 min in digestion buffer prior to extraction removed most exogenous DNA. Pelleting exogenous material led to DNA loss, so exogenous material was not pelleted. Instead, DNAs were extracted from the soaking solutions, followed by a 2-h digestion and organic extraction. Mixtures were common when analyzing autosomal STRs, so Y-STR analysis is suggested. Microfuge tubes were better for transporting fingernail evidence, and there was little degradation after storage for 6 months. These optimizations helped increase exogenous DNA recovery from fingernail evidence.
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