Traditional nail clipping methods for subungual DNA collection have been questioned because they often yield high total quantities of DNA but recover little foreign DNA. This study used simulated scratching experiments to compare three collection methods for recovering foreign DNA from beneath fingernails.

Fifteen female volunteers with fingernails ≥2mm in length performed controlled scratching on one male volunteer's forearms using standardised force (approximately 1kg per finger). Two fingers from each volunteer were randomly assigned to one of three collection methods: (i) nail clippings, (ii) cotton swabs, and (iii) 4N6FLOQSwabs™ Genetics (subungual swabs), resulting in 30 samples per method. DNA was extracted with the DNA IQ™ Casework Pro Kit, quantified with Quantifiler® Trio, and amplified with GlobalFiler™ and Yfiler™ Plus kits.

Nail clippings yielded the highest total human DNA but showed poor foreign DNA recovery, with no reportable male profiles from autosomal STR analysis. This result was expected, as the high levels of endogenous female DNA from the volunteers’ nails would mask foreign male DNA. In contrast, both swab-based methods recovered significantly more foreign DNA, producing reportable male profiles in 46.7% (14/30) of subungual swabs and 36.7% (11/30) of cotton swabs (p<0.001 vs nail clippings). Further analysis using Y-STR amplification was performed on DNA recovered from all three methods. This male-specific STR amplification circumvented the masking effect, resulting in similar recovery (43-53%) of Y-STR profiles across the three methods. However, DNA quantitation revealed that the proportion of male DNA relative to total human DNA was significantly higher for both swab methods as compared to nail clippings.

In conclusion, swab-based collection methods are significantly more effective than nail clippings for recovering foreign DNA from beneath fingernails. Importantly, total DNA yield does not equate with evidential value when foreign DNA recovery is the primary objective. The performance of cotton swabs and 4N6FLOQSwabs™ Genetics were comparable. Based on these findings, swab-based methods are recommended as the primary approach for subungual DNA collection when foreign DNA is the investigative target; nail clippings may be considered as a supplementary option if necessary.

Authors

• Yu Wei Phua (DNA Profiling Laboratory, Biology Division, Health Sciences Authority, Singapore)
• Nicole An Ting Tang (DNA Profiling Laboratory, Biology Division, Health Sciences Authority, Singapore)
• Wai Ling Wong (DNA Profiling Laboratory, Biology Division, Health Sciences Authority, Singapore)
• Noorshahira Mohamad (DNA Profiling Laboratory, Biology Division, Health Sciences Authority, Singapore)
• Hann Qian Lim (Forensics Division, Criminal Investigation Department, Singapore Police Force, Singapore)
• Mohamed Amir Bin Mohamed Arshad (Forensics Division, Criminal Investigation Department, Singapore Police Force, Singapore)
• Christopher Kiu Choong Syn (DNA Profiling Laboratory, Biology Division, Health Sciences Authority, Singapore)