Who packed the drugs? Improved DNA recovery and its impact on findings given activity-level propositions

The interpretation of forensic DNA findings given activity-level propositions depends on the processes governing DNA transfer, persistence, and recovery, as well as on the laboratory methods used to recover DNA. This study investigated the effects of transfer type, substrate characteristics, and DNA extraction strategy on DNA yield, STR profiling performance, and resulting likelihood ratios given activity-level propositions. Direct and indirect transfer experiments were performed using personal bags (backpacks and purses) and zip-lock bags. Three analytical approaches were compared: Chelex extraction, a co-extraction protocol combining the QIAamp DNA Mini Kit and mirVana™ miRNA Isolation Kit, and Direct PCR. Consistent with established transfer principles, direct transfer produced substantially higher DNA quantities than indirect transfer across all methods. The proportion of DNA transferred from bags to zip-lock bags was slightly influenced by bag type and inner surface characteristics. Extraction approach noticeably affected DNA yield and profiling success. The co-extraction protocol generated higher DNA quantities and more robust STR profiles compared with Chelex and Direct PCR. Direct PCR provided advantages in minimizing DNA loss but showed limitations related to inhibition and overall profile quality compared with the co-extraction protocol. Differences in extraction efficiency altered the distributions of DNA quantities observed under direct and indirect transfer scenarios, thereby influencing likelihood ratios given activity-level propositions. These findings are consistent with inter-laboratory studies demonstrating that methodological variation can influence the distributions underlying evaluations of DNA findings given activities. Overall, the results demonstrate that laboratory methodology, particularly the DNA extraction approach, plays a critical role not only in DNA recovery and profiling performance but also in the evidential weight assigned when evaluating DNA findings given activity-level propositions.