PowerPlex® Fusion 6C System versus PowerPlex® Fusion 5C: A Comparison of Performance Metrics

Following the FBI mandated expansion of the CODIS core loci from 13 to 20, several manufacturers developed short tandem repeat (STR) typing kits in response to the new criteria. One such manufacturer was the Promega Corporation, which released the PowerPlex® Fusion 5C megaplex STR typing kit (Fusion 5C) in 2012. Currently, the Virginia Department of Forensic Science (VDFS) utilizes this amplification kit for both casework and database applications. In 2015, Promega released the PowerPlex® Fusion 6C STR typing kit (Fusion 6C), which contains three additional loci beyond those included in PowerPlex® Fusion 5C, and a sixth color channel. The power of discrimination increases with the inclusion of these additional loci. However, this alone does not justify the replacement of Fusion 5C for Fusion 6C, as multiple costly and time-consuming validation studies would need to be performed before any switch. Given all of the processes to be validated and optimized in the VDFS laboratory prior to implementation, to be beneficial, it would need to be demonstrated that Fusion 6C showed several additional advantages in performance beyond an increase in the already large discriminatory power of Fusion 5C. In this study, both STR amplification kits were assessed for performance metrics in several aspects, including baseline noise, sensitivity, pull-up, allele ambiguity, mixture analysis, and degraded sample analysis. This research found that Fusion 6C demonstrated a lower baseline noise level, less ambiguous pull-up (versus a true minor peak) in three- and four-person mixtures, and statistically significant higher allele counts in four-person mixtures compared to Fusion 5C. Fusion 6C also utilizes a shorter polymerase chain reaction (PCR) cycling procedure, taking approximately 60 minutes to Fusion 5C’s 90 minutes. Disadvantages of Fusion 6C include lower sensitivity and more bin overlap. Fusion 5C and 6C are comparable in two-person mixture pull-up and allele counts, three-person mixture allele counts, and likelihood ratios from probabilistic modeling of three- and four-person mixtures. Based on the results, Fusion 6C possesses several critical advantages in addition to the higher possible power of discrimination, and it will be recommended that the Virginia Department of Forensic Science consider implementing this amplification kit for both caseworking and database applications