Internal validation for analytical and stochastic thresholds in the GlobalFiler system

Abstract

 Interpretation of crime stain profiles is one of the challenging tasks of forensic scientists. Before using the probabilistic genotyping system, forensic scientists should interpret crime stain profiles using analytical and stochastic thresholds. According to the guidelines presented by the Scientific Working Group on DNA Analysis Methods, these thresholds shall be based on and supported by applicable internal validation studies. In this study, we performed the internal validation study of our DNA typing system to determine these thresholds. A total of 350 DNA samples and 11 negative controls were amplified using GlobalFiler^TM PCR Amplification Kit with 30 cycles and PCR products were then analyzed on a SeqStudio Genetic Analyzer. As a result, the analytical threshold was set to 110 RFU, which has high sensitivity to obtain almost full profiles of 0.063 ng DNA, and has high specificity not to detect most of the pull-up and drop-in peaks. The stochastic threshold was set to 890 RFU allowing a 1% probability of allelic drop-out based on logistic regression. These thresholds are useful for interpreting crime stain profiles in our GlobalFiler system.