An inter-laboratory comparison of probabilistic genotyping parameters and evaluation of performance on DNA mixtures from different laboratories.

Probabilistic genotyping (PG) is becoming the preferred standard for evidence interpretation, amongst forensic DNA laboratories, especially those in the United States. Various groups have expressed concern about reliability of PG systems, especially for mixtures beyond two contributors. Studies involving interlaboratory testing of known mixtures have been identified as ways to evaluate the reliability of PG systems. Reliability means different things in different contexts. However, it suffices here to think about it as a mixture of precision and accuracy. We might also consider whether a system is prone to producing misleading results – for example large likelihood ratios (LR s) when the POI is truly not a contributor, or small LR s when the POI is a truly a contributor. In this paper we show that the PG system STRmix™ is relatively unaffected by differences in parameter settings. That is, a DNA mixture that is analyzed in different laboratories using STRmix™ will result in different LR s, but less than 0.05% of these LR s would result in a different, or misleading conclusion as long as the LR is greater than 50. For the purposes of this study, we define LR s assigned using different parameters for the same mixtures as similar if the LR of the true POI is greater than the LR s generated for 99.9% of the general population. These findings are based on an interlaboratory study involving eight laboratories that provided twenty known DNA mixtures of two to four contributors and their individual laboratory STRmix™ parameters. The eight sets of laboratory parameters included differences in STR kits and PCR cycles as well as the peak, stutter, and locus specific amplification efficiency variances. • Inter-laboratory study of 155 mixtures and PG parameters from eight laboratories. • PG evaluation of different STR kits, AT values, PCR cycles and stutter values. • STRmix™ returns similar LRs for donors ≳300 rfu template. [ABSTRACT FROM AUTHOR]

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