DNA Mixture Interpretations: A Q&A With NIST’s John Butler

Whether from skin cells, saliva, semen or blood, DNA from a crime scene is often collected and tested in a lab to see if a suspect’s DNA is likely a contributor to that sample or not. But every DNA sample tells a different story, and some samples are easier to interpret than others. The simplest type of DNA profile to interpret is one where the sample includes hundreds of cells from only one person. When two or more people have contributed to a sample, it’s called a DNA mixture. Some mixtures are so complicated that their stories remain a mystery even to the best forensic DNA experts.

John Butler, a Fellow at the National Institute of Standards and Technology (NIST), and a team of authors have recently completed a draft scientific foundation review of the different methods forensic laboratories use to interpret DNA mixtures. The team urged for more interlaboratory participation in studies to demonstrate consistency in methods and the technology used in DNA mixture interpretation, as well as a need for sharing data publicly. In this interview with NIST’s Christina Reed, Butler — who has over 30 years of experience with DNA profiling, is the author of five books on the subject, and has led training workshops on interpreting DNA mixtures — answers some basic questions about the importance of this fast-growing field of forensic science.

What’s the concern here? Why is NIST reviewing how labs are conducting DNA mixture interpretations?  

Recent advances in technology and methods have allowed labs to test DNA samples that contain fewer and fewer cells and come from multiple sources. If single-source DNA is like basic arithmetic and a two-person mixture is like algebra, then a complicated mixture is like calculus! Each advance builds upon the fundamental properties of the other less-complicated concept and requires additional training and perspective to navigate. In our review, we discuss the importance of understanding the underlying principles in order to better appreciate the limits. We wanted to see if there were established methodologies that worked better than others when tested, and where those limits were being drawn. What we found is that there is not enough publicly available data to enable an external and independent assessment of the degree of reliability of DNA mixture interpretation practices.

Why is it important that this data be public? Why is transparency and openness important in science?

An important part of science is sharing results, so that others may test the methods themselves and see if the results are reproducible. DNA testing has 35 years of scientific history. People have confidence in the results from DNA testing of a single source or simple mixture sample because rigorous studies have demonstrated that this type of testing produces consistent and reproducible results. Ideally, complicated DNA mixtures should undergo the same level of scrutiny to confirm their reliability.

Why are forensic scientists now frequently dealing with casework that requires analyzing DNA mixtures in their labs?

People constantly shed small amounts of DNA into the environment. By touching something or someone else, coughing, or even speaking, people can potentially transfer small amounts of DNA from one surface to another. For example, if I shake your hand and touch a table, I may transfer some of your skin cells to that surface. With improvements in DNA testing methods, we don’t need much DNA to make a profile and see perhaps if I am a likely contributor to that sample or if you have contributed — even if you never touched the table directly. That level of DNA profiling is useful for many different types of crimes, but also brings up the issue of relevance. We aren’t explaining how DNA got to a location. The analyst isn’t answering the question “Is this person’s DNA on this sample?” with a yes or no; they provide instead a likelihood ratio that indicates probability. If the DNA mixture is too complicated, they might also say, “I can’t tell.”

What can an analyst do to deal with complications?

To interpret a DNA profile, the DNA analyst uses their judgment, training, experience and tools, including computer software, and considers factors such as case context. You have to understand the limitations of your system, and that is possible by testing it with known DNA samples that are as complicated as the ones you are trying to interpret. This helps to identify the edges of your system, and not just for software. If you know what questions you are trying to answer and you ask yourself, “Do I have the information I need to answer this?,” sometimes you don’t, and that’s when you know you’ve reached your limit.

What do you hope this report will accomplish?

An important part of science and advancing any field in science is sharing results with others. If we increase our understanding of the causes of variability among laboratories and analysts, then we can help determine where improvements are needed and help increase confidence in DNA mixture interpretations across the entire community. Labs can help improve confidence by collecting their data and sharing that data, especially the data used in performance checking. That’s when you know the correct answer and you test the performance of the models with data similar in complexity to the case situation that’s being considered. Making that data available and explaining it will increase confidence. Then people can say, “The reason I have confidence in these results is that I have tested my system against samples that are similar in complexity to this one and the system produced reliable results.”