Comparison of different methods of DNA recovery and PCR amplification in STR profiling of casings—a retrospective study

Casings represent common evidence in a forensic laboratory, due to high frequency of firearms usage during perpetration of criminal offenses. Possible DNA evidence from casings is compromised by degradation, inhibition, and initial low-quantity deposition of biological material. For that reason, in the last 15 years, scientists have been trying to optimize procedures for recovery and amplification of DNA possibly present on its surface. In this study, we share our 12-year experience done on a total of 698 casework casings, comparing two DNA recovery methods commonly used—soaking and swabbing, as well as efficacy of two commercially available DNA amplification kits (AmpFLSTR® Identifiler® and AmpFLSTR® Identifiler® Plus kits). Of all analyzed casings, 30 were excluded as 28 (4%) matched the victims’ DNA profiles and 2 (0.3%) samples were proved to be contaminated by technicians. Overall success in obtaining interpretable DNA profiles was 15.6% (104/668) (13.8% (55/399) for AmpFLSTR® Identifiler® Plus combined with soaking, 22% (33/150) for AmpFLSTR® Identifiler® Plus combined with swabbing, and 13.4% (16/119) using AmpFLSTR® Identifiler® kit and swabbing recovery method). Our data suggest the importance of both DNA recovery methods and amplification kits used, and point out swabbing of casings combined with AmpFLSTR® Identifiler® Plus kit as methods of choice. Nonetheless, our results are based on real casework and are prone to uncontrolled variables.