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Trace DNA From Fingernails: Increasing the Success Rate of Widely Collected Forensic Evidence

NCJ Number
Date Published
December 2015
64 pages
This project compared multiple methods for isolating exogenous cells from an offender's biological material trapped under a victim's fingernails from scratching the offender, so as to purify DNA from these cells and produce short tandem repeat (STR) profiles.
Overall, the study identified the strengths and weaknesses in each step of fingernail evidence processing, resulting in recommendations that should be useful to forensic examiners. The study first placed a set amount of male blood on female fingernails, allowing for quantification of exogenous cell recovery based on a Y chromosome assay. Then multiple aspects of fingernail evidence processing were examined, including fingernail evidence-collection methods used by practitioners (clipping nails, swabbing nails, or scraping nails); DNA isolation (standard organic extraction versus a commercial kit); and DNA analysis (autosomal STRs versus Y-chromosome STRs). Based on these results, a protocol was developed and applied to scratchings. This involved females scratching male volunteers on the forearm using a set amount of force. Scratchings were processed using the most effective procedures. The prevalence of cell loss or cross-contamination using certain nail processing techniques was also examined, as was cell loss and cross-contamination during nail transport and the influence of nail polish. Results showed that the three methods for collecting exogenous DNA from nail evidence produced statistically significant differences in DNA yields, with the soaking method generating the most exogenous DNA; however, it also generated the most endogenous (nail) DNA. When Y-STR analysis of those DNAs was conducted, strong exogenous profiles were produced. Transporting and processing nails together, as is often done in a forensic setting, was advantageous; although no cross-contamination occurred during transport, it did occur during nail processing. Exogenous cells were lost during both procedures. 15 tables, 7 figures, and 13 references

Date Published: December 1, 2015