Validation and assessment of the Investigator® 24plex QS kit for forensic casework application: Comparison with the PowerPlex® fusion system and GlobalFiler™ PCR amplification kits

Casework evidence samples are likely to be placed under diverse and harsh environments as compared to quantified DNA samples including serial-diluted standard DNA samples. Internal validation of a novel STR kit using casework evidence sample, which is conducted according to various conditions such as DNA contamination and degradation, is crucial before being used as a forensic application. Therefore, this study aimed to elucidate the reliability of the Investigator® 24plex QS kit through DNA derived from casework evidence and to assess whether it is applicable to STR analysis together with PowerPlex® Fusion System and GlobalFiler™ PCR Amplification Kit. DNA was extracted from 189 casework evidence samples in a total of 77 cases. The mismatch of the allelic size of this kit through allelic sizing precision test, was suitable according to ENFSI guidelines. All heterozygous balance of the three kits were above 0.6 recommended value of ENFSI guideline. The number of allele drop-in was most frequent in the GlobalFiler™ PCR Amplification Kit. In addition, the number of allele drop-out was most frequent in the Investigator® 24plex QS kit. The cutoff concentration of DNA detected in three kits of one complete STR was approximately 45 pg/μL on average. Despite of several limitations, the Investigator® 24plex QS kit is considered to have the capability to be used for STR analysis of casework evidence samples.