MIX13 Interlaboratory Study on DNA Mixture Interpretation

August 2, 2018

In 2013, the National Institute of Standards and Technology (NIST) conducted a study in which 108 forensic laboratories interpreted the same set of DNA evidence so the results could be compared across labs. These types of collaborative exercises, called inter-laboratory studies, help participants understand the sources of variability in their test results and can lead to best practices and other improvements that raise performance for the entire field.

This study focused on DNA mixtures, a type of evidence that, in some cases, can be particularly difficult to interpret. DNA mixtures, which occur when the evidence contains DNA from two or more people, have become more common in recent years as forensic DNA methods have become more sensitive. Increased sensitivity allows labs to analyze evidence from a greater variety of crimes, but it also increases the likelihood that a lab will encounter challenging DNA mixtures.

The study found that when labs interpret particularly complex DNA mixtures, their results can vary significantly across labs. After the study was completed, NIST shared the results at public conferences and workshops so that participants could understand the causes of variability and improve their methods. A summary of the results has also been publicly available on NIST’s forensic DNA-focused website, STRBase, since 2014.

While one of the main objectives of the study was to provide useful feedback to laboratories, NIST also received requests that the study be published in a peer-reviewed journal. A paper describing the study, including test results and lessons learned, was published this week in FSI: Genetics.

The study was not designed to rigorously measure laboratory performance. For example, some labs did not have a second analyst review the test results as they would in real cases. Therefore, the results may not reflect error rates in actual casework. However, this study does highlight the need for continuing efforts to help ensure that laboratories produce consistent and reliable results when analyzing complex DNA mixtures.