DNA Mixture Interpretation Principles: Insights from the NIST Scientific Foundation Review

DNA mixture interpretation has become more challenging in recent years due to several factors including submission of more touch evidence samples to aid investigations and generation of more sensitive DNA test results with new STR typing kits. This sensitivity enables recovery of DNA results from low quantities of biological material. However, the PCR signal amplification process, which provides the DNA test sensitivity, also introduces artifacts into the resulting data including stutter products as well as allele drop-out, allele drop-in, and heterozygote imbalance that occur due to stochastic effects. These artifacts combined with allele sharing among contributors to the mixture make pairing contributor alleles into their corresponding (donor’s) genotypes more difficult and influence uncertainty associated with the mixture interpretation process. Probabilistic genotyping software programs are being implemented in many laboratories to aid evaluation of low-template DNA results either from single-source samples or trace components of mixtures.

The National Institute of Standards and Technology (NIST) has been Congressionally-funded to perform scientific foundation reviews of select forensic disciplines. These reviews are intended to establish what is well-known and well-supported empirically in a forensic field as well as where gaps may exist that need further study. DNA mixture interpretation was selected as the initial NIST scientific foundation review given the existence of abundant literature and a need expressed by members of the community. Multiple interlaboratory studies have noted variability among accredited laboratories using validated approaches to the same DNA mixture data.

For the past year (since September 2017), a NIST review team has been studying issues around DNA mixture interpretation. An important goal of this project is to identify, consolidate, and share core principles and supporting publications with the community to encourage deeper learning and understanding of DNA mixture interpretation. More than 500 articles related to DNA mixture interpretation have been gathered and are being examined to better understand capabilities and limitations as reflected in the scientific literature. An external DNA Mixture Resource Group, composed of 13 experienced practitioners, has provided valuable input and feedback to the NIST team on a regular basis. Status of this study will be shared along with primary considerations identified.