Since the first use of DNA evidence in a criminal case in 1986, forensic scientists have considered biological material to be relatively reliable physical evidence.
While early technology required a substantial amount of biological material to extract enough DNA to build an individual profile for analysis, researchers have since discovered that they can obtain reliable DNA from more than just bloodstains or visible fluids; they can also obtain it from “touch DNA” that is left behind on surfaces or objects. Although touch DNA can be essential for forensic casework, it also comes with its share of issues, including those related to:
- Low quantity of useable DNA,
- High variability in the amount of DNA left by touch; that is, high variability in the amount that one person leaves, and high variability in the amount left from person to person, and
- DNA degradation, including the many factors that can cause DNA to break down over time.
The results from rigorous analysis of these complicated factors have important implications for how touch DNA is collected, analyzed and interpreted.
In 2018, the Forensic Technology Working Group at NIJ called for “comprehensive, systematic, well controlled studies that provide foundational knowledge and practical data about ‘touch evidence’ persistence in the real world.” That same year, Meghan Ramsey’s group at MIT Lincoln Laboratory began quantifying how long touch DNA would persist on certain surfaces under specific conditions. Building on that knowledge, and in collaboration with Ramsey, scientists at South Dakota State University created predictive models of how DNA degrades on different surfaces under a range of environmental conditions.
Testing for persistence
In their study, the researchers addressed two central questions: How do surface type, environmental condition, and exposure time affect the stability of touch DNA evidence? and does the stability of touch DNA samples differ from control DNA samples?
To address these questions, scientists deposited control DNA and touch DNA samples onto steel bolts and cotton fabric swatches. Then, they examined the DNA residue over time, across varying temperature and humidity combinations, and under UV light exposure.
Researchers measured both the amount of DNA and the quality of DNA using a degradation index. As expected, the amount of DNA that persisted on the steel and cotton decreased over time. But, the amount of DNA left by touch varied more than in the control samples. Additionally, of the varying environmental factors, UV light had the largest effect on DNA degradation on both steel and cotton.
To predict the amount of DNA degradation over time, Ramsey worked with her collaborators to fit the DNA degradation data (based on temperature and humidity exposure) to a linear, mixed effects model. The mixed effect model showed DNA degraded more in high temperature and low humidity. Conversely, DNA samples were more stable at low temperatures.
To further examine DNA degradation, Ramsey and colleagues compared the completeness—whether the DNA profiles could be submitted to a database for a potential match—of two DNA profiles: environmentally exposed touch DNA recovered from steel bolts and unexposed reference sample DNA from cheek cells.
The data showed that for all samples exposed to UV light, the researchers could not determine the DNA profiles. In other words, DNA samples exposed to UV light were too highly degraded to be useful in a forensic analysis. Under most conditions, all samples deposited on steel were highly stable. A noteworthy exception occurred under UV light exposure. Additonally, the results suggest the completeness of the DNA profiles determined from the touch DNA samples compared with cheek swab samples varied substantially across all environmental conditions.
Is there not enough DNA or is it degradation?
Throughout the course of this research, low and variable quantities of touch DNA collected remained a challenge; the low quantities of the initial touch DNA that scientists could recover made it difficult for researchers to evaluate the level of DNA degradation properly. Future work aims to increase the initial amount of touch DNA collected to record its degradation more accurately over time.
Still, those in forensics and law enforcement can glean valuable information from this ongoing research regarding the persistence of DNA in certain environmental conditions. For instance, investigators are more likely to recover useable DNA in cool and dry indoor environments than hot and humid outside conditions. Moreover, they may have better success obtaining DNA from stainless steel objects than fabric.
“The results from the study have generated a number of recommendations for best practices that the forensic science community can use to interpret and evaluate touch DNA evidence in a laboratory setting,” said Tracey Johnson of the National Institute of Justice.
Collectively, these studies provide the most comprehensive information to date on the persistence of touch DNA evidence.
Republished courtesy of NIJ. This article is based on the grantee report, “Persistence of Touch DNA for Forensic Analysis” by Meghan Ramsey.