DNA is Too Sensitive to be Infallible

Recently my local paper ran the article, “A Major Shake-Up for Legal Landscape,”1 reporting on the unintentional transfer of DNA evidence to a crime scene by paramedics. The paramedics had used a finger pO2 device on an unconscious male while transporting him to the hospital. Subsequently, on a later call to a home burglary that turned into a homicide, the same pO2 reader was placed on the corpse. The forensic lab performed DNA amplification on fingernail scrapings. This produced a match with a local person with a long rap sheet. He became the prime suspect. Presto—an indictment!

The DAs jumped on this evidence that they say placed the suspect at the scene of the crime. After all, they, too, watch popular TV shows. Transference of DNA or sample contamination is recognized as possible, but is usually dismissed by juries. However, the prime suspect had an apparently airtight alibi: He was unconscious in a hospital about 10 miles away at the time of the murder. The prosecutor ignored this evidence and won a conviction. In California, some believe that the system is rigged to motivate police and prosecutors to win convictions since it is part of their performance evaluation, and that the State of California pays incentive payments to police departments and DA departments for convictions. Thus there is an incentive to focus on convictions rather than justice.

Transference can be easy, with skin flakes, psoriasis, etc., contaminating scenes or transferred when giving change and possibly even when shaking hands. Semen can be transferred to other garments if they are in the washer together. One attorney is quoted as saying that “you could very innocently come in contact with an object, and days or months later, your DNA shows up at a crime scene you had nothing to do with.”

The problem, of course, is that DNA amplification has a very low detection limit due to polymerase chain reaction. The proper interpretation is that differences of short tandem repeats (STRs) should rule out matches, but matches should raise questions of possible reasons. Sample contamination? In the laboratory? Dry labbing? Transmission by money? Shaking hands? Legitimate prior occupation of the sample scene, such as a hotel room? As the STR databases increase, so does the risk of finding a match that has an explanation other than being present at an event.

So, as scientists who might survive the jury selection process, it is important to recognize that the science of DNA testing has limitations. Assurances from police, labs, and prosecutors that the evidence was unambiguous should be treated skeptically. Other evidence should corroborate it. Finally, one needs to be aware of potential extra case considerations such as bounties or promotions paid for successful prosecution. Follow the money.


  1. Kaplan, T. Major Shake-Up for Legal Landscape. San Jose Mercury News, June 29, 2014, p 1A.

Robert L. Stevenson, Ph.D., is Editor, American Laboratory/Labcompare; e-mail: rlsteven@yahoo.com.

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