In a forensic laboratory, achieving both efficiency and accuracy can be a hard balance to strike. Add in the outside politics, the emotional toll the work can take and the daily drudgery of a 9 to 5, and its understandable that, sometimes, it can all feel like too much.
But forensic scientists play a critical role in our criminal justice system. The stakes aren’t just missing a deadline, rather, it comes down to life and death.
“Analysts in this field have very special skills and talents that others don’t,” said Nancy Dinh, a forensic DNA analyst at Forensic Analytical Crime Lab (California).
At Forensic Analytical Crime Lab, Dinh works for the defense, assisting attorneys in understanding results from outside laboratories. The other half of her work involves reexamining items and analyzing evidence that was not looked at before to determine if any assertions brought on by the prosecution can be supported by DNA results alone.
In a presentation at AAFS last month, Dinh shared a few interesting case examples that reveal the value of specimen-specific examination and sampling techniques. The takeaways from each case shared are critical lessons on the accuracy of forensic DNA analysis.
“Automation has overtaken our laboratories,” said Dinh. “As a result, the quality of work in some cases could be compromised or an interesting detail could be overlooked simply because our end goal seems to have shifted from finding truth in evidence to doing just enough to make someone happy. Ultimately, the goal is to find truth in DNA evidence.”
Case 1: Sexual assault of a minor by family member
In 2015, a female minor alleged her grandfather had sexually assaulted her. The laboratory that did the initial analysis used microscopy to analyze a small cutting taken from the inside front crotch area of the girl’s underwear. No semen nor acid phosphatase (AP) was detected. Although the lab did take additional small cuttings, they were never examined microscopically since almost all DNA results came back to the victim. However, a small amount of male DNA was detected, so the lab proceeded with Y testing. Results matched the grandfather, as well as all males in the bloodline.
While the grandfather initially denied the allegation, he then changed his story to say it was his son—the girl’s father—who committed the assault.
Dinh was then asked to retest to confirm the absence of semen and generate a stronger autosomal profile to eliminate the grandfather as the abuser. But, Dinh did find AP activity in a small stain that was not previous cut from the underwear. She did not find any sperm cells in the stain from the upper part of the crotch, right by the previous lab’s screening sample. There was, however, sperm in the stain from the middle of the crotch, a spot right by were the previous lab sampled but didn’t examine microscopically. Dinh also detected semen in the underwear. Subsequent autosomal testing revealed the grandfather as the match.
“When you screen an item, make sure your screening results come from your actual testing sample, as well. The goal is any investigation is to uncover the real story, and if you’re not careful, you can miss crucial parts and ultimately deliver incomplete and maybe even a false narrative behind the evidence,” Dinh said.
Case 2: Sexual assault/murder
In 1986, the body of a female victim was found, sexually assaulted and murdered. Even though a sexual assault kit was collected from the deceased victim, it was not tested until 2003. The lab detected semen on the victim’s vaginal swab, but it was determined to be ineligible for CODIS. At the time, semen was not detected on the victim’s anal swab specimen. Retesting in 2004 and 2016 did not include the anal swab.
In 2020, the kit was sent to Dinh for a fourth round of testing.
“I opened it up and all that remained was a nose swab and some empty vials—one of which was labeled anal swab. I noticed there was residue inside the vial so I decided to give it a shot, even though previous labs said there was no semen on the swab itself. If there’s anything I have learned from my years of retesting items is it doesn’t hurt to double check,” Dinh said.
Testing the residue sample left in the vial Dinh discovered sperm from the cell debris, indicating there was in fact semen on the initial anal swab. DNA testing revealed a mixture between at least two males, a major and minor contributor. Dinh is still waiting the outcome of a CODIS search.
“Just because a parent specimen or substrate is not there, all hope is not lost,” she said.
Case 3: 2018 homicide, semen on the gun
In 2018, a man was gunned down in a car by multiple suspects who fled the scene but abandoned their guns, which were later recovered by police. One suspect was identified from the DNA left on gun #1. But DNA left on gun #2 was a complex mixture of four people, with two significant contributors. The initial lab was able to exclude two individuals, but they couldn’t draw any conclusions regarding other potential contributors.
Dinh received the second gun to retest last year.
“The previous lab did a general swab of the entire gun, which is fine in theory if you’re just looking for biological from the gun in general. But in my experience, gun samples always result in heinous mixtures. So my preference is to sample different parts of the gun separately to minimize the chance of getting complicated or uninterpretable mixture DNA profiles,” Dinh explained.
Dinh took a sample from the trigger and put it under the microscope. While that is not customary for gun casework, she says she carried over the habit from training. Turns out, her habit was case-breaking. Dinh found a ton of semen on the trigger. When she processed it, the result was a robust unique profile. The man was known to the police but was not a suspect.
“Use past experiences with similar items to guide your testing,” Dinh said. “Even though microscopy isn’t typical, it’s still a powerful tool. It’s time consuming but you never know what you may find.”
Like semen. On a gun.
Case 4: Homicide with firearm, one DNA source?
In 2016, a victim was fatally shot with a revolver, which was left and recovered near the crime scene. The initial lab sampled both sides of the gun’s grip, and three other small areas that had texture. No DNA was recovered from the grip, and only 0.17 nanograms of DNA was recovered from the other samples. The lab ultimately concluded there was no evidence of more than one donor, however, the number of donors could not be conclusively determined.
Based on that confusing conclusion and other review findings, Dinh was asked to retest the gun in 2017 to see out if anyone else’s DNA was present.
“Since I was not trying to identify a potential user, I did a global swabbing of the gun,” Dinh explained. “Generally, when I take a swab, I scrub it as if I am cleaning it, no matter how many swabs it takes. I recovered approximately 3 ng of DNA from swabbing.”
The DNA result revealed a complex mixture of at least four people. Dinh said it somewhat resembled the previous lab’s results, with slightly different proportions and more robust peaks.
“Unless there was a surplus of DNA in every spot except where the previous lab sampled, this example demonstrates the important of rigorous swapping and how that can affect the quality of your results,” the DNA expert said.
While these are just a few case examples, Dinh says there are dozens of cases where she has retested evidence only to find the exact opposite results of previous testing.
“It could be inadequate training, experience, unreasonable outside pressures, limited resource, etc., but I hope we can take a step back and evaluate our lab practices and think about how we can all improve the way we do work to minimize these kinds of errors. It’s our responsibility to help maintain DNA analysis as the gold standard.”