Impact of Semi-Permanent Nail Polish on Forensic DNA Profiling and Phenotyping from Fingernails

Background/objectives: The increasing global trend in nail beautification may lead to analyses of nails with semi-permanent polish for the identification of degraded human remains. This study aimed to evaluate the effects of cosmetic nail treatment on forensic STR DNA profiling and phenotyping of eye, hair, and skin colour characteristics using a massively parallel sequencing (MPS) assay. Methods: Forty-two nail samples obtained from 21 volunteers, classified in "new", occasional and regular semi-permanent polish users, were submitted to DNA analysis. Results: The use of semi-permanent nail polish, particularly when applied repeatedly, resulted in a significant reduction in DNA recovery, but it did not affect STR typing for personal identification. Mixed STR profiles were observed in 28.6% of the samples, indicating that the nail washing procedure employed before DNA extraction did not completely remove the foreign DNA; however, this could be useful depending on the forensic context. FDP analysis was successfully applied on nails with semi-permanent polish that showed a good quantity of DNA and single-source profiles. Conclusions: The results highlight the evidentiary value of fingernails even if treated with semi-permanent nail polish that should still be regarded as a source of DNA for personal identification and further investigation in the forensic context.