Verification testing in a multi-laboratory system using ANSI/ASB Standard 020

Protocol verification is a fit-for-purpose test of laboratory procedures. We present here verification testing of a DNA mixture interpretation protocol, following ANSI/ASB Standard 020, Standard for Validation Studies of DNA Mixtures, and Development and Verification of a Laboratory's Mixture Interpretation Protocol. The blind testing called for in the standard was performed on a set of nine DNA mixtures created with contributors unique to the verification, using a range of donor ratios (distinguishable and indistinguishable), DNA inputs (0.25-3.6 ng), and numbers of contributors (2-4). The testers ("verifiers") were given .hid files, along with limited contextual information that simulated the state of caseworker knowledge prior to PCR amplification, and they were tasked with determining contributor number and suitability for interpretation, analyzing each interpretable mixture, and generating simple likelihood ratios (LRs) and corresponding verbal predicate assignments. Although the differences observed across verifiers were within the scope of the draft mixture interpretation procedure and resulted in non-consequential differences among the calculated LRs, we found that the process led to significant improvements in training efficiency and pre-release protocol refinement. We also found that DNA mixture selection, verifier training prior to verification testing, and assessment criteria development must all be considered carefully to make the process as effective as possible, particularly in a multi-laboratory system. The planning and results summarized in this paper can serve as a template to other forensic DNA laboratories seeking to incorporate the recommendations of ANSI/ASB Standard 020 into their quality assurance systems.