Internal validation of latest generation STR kits for direct STR typing from reference samples

Abstract

Forensic STR kits are manufactured by various biotechnology companies. Differences in primer designs and proprietary chemicals, as well as the availability of instruments in each laboratory, may result in performance differences. As such, internal validation is a necessary step prior to adopting a new kit, especially if accreditation is sought. Here, we evaluated three latest generation direct STR kits (Thermo Fisher Scientific’s GlobalFiler® Express, Promega’s PowerPlex® Fusion 6C, and Qiagen’s Investigator® 24plex GO!) for direct STR typing of reference samples as part of ISO17025:2017 implementation. Thirty buccal swab samples (untreated cotton swabs) were used to optimize the parameters that could affect test results, such as the amount of swab used, PCR cycle number, and electrophoretic injection condition. Each kit was then used for direct STR typing of additional one hundred untreated buccal swabs. A scoring system based on profile quality was used to evaluate the kits: high-quality, minor-issue, moderate-issue, and critical-issue. GlobalFiler® Express kit showed the highest number of high-quality profiles (i.e. full profile with no quality flag), followed by PowerPlex® Fusion 6C and Investigator® 24plex GO!. Non-standard loci typed in the kits (e.g. DYS391 and “Quality Sensors”) provided additional information in cases where amelogenin-Y failed to amplify and also where inhibitors were present. In summary, we selected the GlobalFiler® Express kit for direct STR typing of untreated buccal swab samples due to its superior performance.