The Evolution of Touch DNA: Invisible Traces at Crime Scenes

In the landscape of forensic science, one of the most transformative developments has been the advent and refinement of touch DNA analysis. This cutting-edge technique, which allows forensic experts to extract and analyze genetic material from the mere remnants left by a person’s touch, has broadened the usefulness of biological evidence in criminal investigations.

Up until the late 20th century, forensic DNA analysis largely relied upon collecting bodily fluids. But the use of polymerase chain reaction (PCR) technology made it possible to analyze the small amounts of skin cells left behind by a person’s touch. DNA extraction and amplification techniques improved further in the early 21st century, with more sensitive instruments leading to wider use. Today, touch DNA is firmly in the mainstream.

Touch DNA is not a magic wand. Even with today’s sophisticated technology, not every touch DNA analysis leads to a conclusive result, and investigators and forensic scientists face challenges in evidence collection, processing, and interpretation. But touch DNA is here to stay, and its evolution is far from finished.

Read on to learn more about the benefits, challenges, and future of touch DNA analysis.

Meet the Expert: Robert O’Brien

Robert O’Brien is the forensic biology section lead at the Global Forensic and Justice Center (GFJC) at Florida International University (FIU). He conducts forensic biology research, including test plan design, and performs experiments to evaluate equipment and techniques used in biological collection and DNA analysis.

O’Brien has developed and delivered training for new technologies, including Rapid DNA instrumentation, and advises operators on tests and techniques available for field use in biological sample detection and screening. He also develops curricula for forensic DNA and serology training programs, delivering instruction in person and remotely.

Before joining GFJC in 2007, O’Brien served as a crime laboratory analyst in the biology section of the Florida Department of Law Enforcement (FDLE), where he supervised and trained other forensic analysts. He has been granted access to input data into the Combined DNA Index System (CODIS) and is qualified as an expert witness in DNA analysis.

The Benefits and Challenges of Touch DNA

“Over the years, touch DNA has changed largely because manufacturers have made more and more sensitive equipment,” O’Brien says. “Before, most labs wouldn’t even consider processing what we process today because they wouldn’t expect to get a result.”

Modern methods of touch DNA analysis can be highly effective, with applications in investigating everything from major crimes to minor thefts. But it’s so applicable that many investigators now send samples to the lab for analysis merely in the hopes of getting a result —and that can lead to backlogs. At the same time, the results of touch DNA analysis are not always straightforward. As instruments have higher and higher sensitivity levels, they’re more likely to pick up DNA from other individuals who have handled the same item.

O’Brien offers a hypothetical case example where, in a gas station robbery, eyewitness statements say the masked perpetrator’s ungloved hand touched the countertop in front of the cashier. Investigators could swab that area of the countertop where the ungloved hand touched, but the sampled area would likely include DNA from several individuals. That could lead to a complex mixture that’s hard to sort out. Which DNA belongs to people who visited the store lawfully, and which belongs to the perpetrator? The risk of mixed profiles and contamination underscore the importance of context and the process by which a sample is collected and processed.

“When a sample comes to the lab, the lab will look at what was collected and then typically speak to the investigator who collected it,” O’Brien says. “It helps to find out why this sample was collected and from where. Was it in an area of general use, where a lot of people might have touched it?”

These concerns materialized in the 2012 case of Lukis Anderson, a man wrongfully charged with murder based on touch DNA evidence. Anderson’s DNA was found on the fingernails of the murder victim, despite him having a solid alibi: he’d been hospitalized for severe intoxication at the time the crime was committed. Only later was it learned that the paramedics who had responded to the scene of the murder had also treated Anderson earlier in the night, inadvertently transferring trace amounts of his DNA in the process. Anderson was exonerated, but the challenges of touch DNA remain.

How Forensic Scientists Work with Touch DNA

After a sample is collected and sent to a lab, forensic scientists will seek to extract DNA from the sample, quantitate it, and then amplify it via PCR before putting it into a genetic analyzer. Genetic analyzers are relatively large instruments, and the most expensive instrument in the process.

The most common method used in genetic analyzers is capillary electrophoresis, where the amplified DNA samples are injected into capillaries filled with a polymer. An electric current is then applied, causing the DNA fragments to move through the polymer at speeds proportional to their size. A laser detects the fragments as they pass a specific point, and the data is used to generate a DNA profile.

Much of this process is automated and not particularly labor intensive. But it does require strict attention to detail, O’Brien says.

“One plate will typically hold 96 samples,” O’Brien says. “So if you’re doing a full plate, then you have 96 different wells that you’re pipetting into, and you’re working with very small volumes, so you have to be very careful.”

All forensic scientists in this area will need a degree in a hard science like biology or chemistry. The educational requirements are set in stone, especially for working with the FBI and their DNA database. Skillswise, one’s process must also be immaculate, as accurate pipetting and general clean lab techniques are crucial to the integrity of one’s analysis. O’Brien highlights attention to detail as particularly important. But in a field where everyone is of high intelligence, he also cautions his trainees in FIU’s DNA training program against becoming overly prideful.

“You must be willing and ready to admit when you make a mistake,” O’Brien says. “It’s so important. When you get to casework, everything you do could be going to court, and you may have to testify. So if you make a mistake, you have to be willing to admit it and learn from it. You can’t place the blame anywhere else.”

Future of Touch DNA

The technology around touch DNA will keep getting better: more reliable, more sensitive, and more applicable in processing samples that were, at one point, unprocessable.

One way in which that’s already occurring is through the rise of Rapid DNA analysis. This type of portable, on-scene test doesn’t require a forensic scientist. While it is generally only applicable in relatively straightforward circumstances, it does reduce the overall burden on labs and forensic scientists, allowing them to focus on more complex cases. Rapid DNA analysis may not require a forensic scientist on-site, expert forensic scientists are perfecting the process itself: GFJC is home to the nation’s first Rapid DNA Center of Excellence.

Another example of advancing touch DNA technology is the way next-generation sequencing (NGS), which is the kind used with services like 23andMe, has come to the forefront. Scientists can better sort out complex low-level mixtures by sequencing the DNA itself. This has enormous potential but is still expensive and time-intensive, so it remains largely the domain of private laboratories. But, over time, the associated costs will likely come down and bring NGS into even wider use.

“There are also high-level mixture deconvolution programs that can take complicated mixtures and sort them out,” O’Brien says. “Basically, they use mathematics that no human could easily do. A lot of labs are using these programs now, and they can process four- or five-person mixtures.”

To be involved in DNA analysis as a forensic scientist is to be part of a dynamic field. This is an area growing increasingly automated, with the more repetitive and laborious aspects trusted to algorithms and robotics. That automation comes with a price: many forensic biologists need to pick up skills in greasing O-rings and managing other technical components—skills not necessarily taught in the traditional undergraduate biology program.

“We’ve been doing more or less the same process for years, but it’s becoming a lot more automated due to the amount of cases requiring DNA,” O’Brien says. “So if you’re a lab person who likes doing everything yourself, you’re going to have to learn how to spend more of your time running robots and maintaining the instrument that’s doing the work.”

But no matter how technologically sophisticated touch DNA analysis gets, it will require a human element. O’Brien highlights the importance of thinking through the context of how a piece of evidence was collected, deciding where to sample from it, considering the behavior of the person whose DNA is being analyzed, reading on-scene reports, and reconstructing the crime in one’s mind. None of that can be rushed or automated—it’s the investigative mindset.

“It’s an ever-changing field, but what hasn’t really changed is the front end, where you have to look at a sample, examine a piece of evidence,” O’Brien says. “That’s where great attention to detail has to come in.”