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How to Swab for Touch DNA Evidence

Posted on Sep 5, 2014 9:23:00 AM


Touch DNA, also known as Trace DNA or Low Copy Number DNA, is an advanced and fairly new crime scene investigation technique used to collect and analyze microscopic transfer cells left behind during the commission of a crime.

From our standard cotton tipped swabs to our self-saturating trace DNA collection device, Puritan Medical Products has a host of DNA-Free forensic swabs appropriate for collecting Touch DNA samples. But why is Touch DNA analysis so important? 

How Touch DNA Works & Why it Matters

This technology has been called a breakthrough by many in law-enforcement for its ability to derive evidence where there is a lack of visible DNA (such as blood, semen, hair, or saliva). It can also be used on fingerprints that are too smudged or incomplete for fingerprint analysis.

Finally, many cold cases whose samples were too small or degraded to prove useful are now resubmitting evidence to labs for Touch DNA analysis.

The process of extracting Touch DNA for forensic analysis involves swabbing, taping, or scraping for trace amounts of epithelial cell-tissue from surfaces like doorknobs, countertops, windows, and even clothing and food. This starter DNA is then amplified using Polymerase Chain Reaction technology to create identical copies that are large enough for proper analysis.

According to Ryan Forensic DNA Consulting, “the amount of DNA needed to yield a full DNA profile with most commercially available amplification kits is approximately 1 nanogram (ng) of DNA and partial profiles can be obtained with even less starting material."

That comes out to about to the infinitesimally small number of about 5-30 skin cells. With those numbers in mind, it's easy to see why Touch DNA has been embraced by police departments across the country. Of course, if you want Touch DNA evidence to work for you, it's essential you adhere to specimen collection best practices to avoid compromising a sample. 

How to Swab for Touch DNA

  1. Be sure to wear appropriate personal protective equipment.
  2. If using a dry swab, extract distilled water from vial using sterile pipette and apply one drop to the side of the tip. Do not use more than one drop, and do not dip the swab into the water.
  3. If using a self-saturating foam-tipped swab, open and remove swab holding foam tip facing down. Then squeeze handle until it pops, releasing medium into tip.
  4. Apply tip to sample area and rub using moderate pressure while rotating the swab to ensure the entire swab surface has made contact with the object.
  5. To avoid compromising the sample by redepositing specimen, do not rotate more than once.
  6. If using dry swab, allow to air dry then place tip in dry transport tube or vial.
  7. If using self-saturating foam-tipped swab, gently peel foam tip away from handle and place in sterile container for transport.
  8. Label the tube or envelope with identifying information.
  9. Use evidence tape to seal outer packaging. Initial and date for chain of custody verification.
  10. Store swab at room temperature or transfer to freezer until testing.

One of the main benefits of Touch DNA technology is that it is so sensitive as to be able to place a suspect at the scene of a crime with just a few skin cells extracted from a forensic swab or scraping. Unfortunately, this is also one of the potential drawbacks. Because the tests are so sensitive, crime scene investigators must use care to avoid contamination.

DNA Evidence Collection Flowchart

How to Avoid Contamination of Touch DNA

Forensic Magazine has eight rules to minimize Touch DNA contamination at a crime scene: 

  • "Limit access to the crime scene or evidence.
  • Do not talk over the evidence.
  • Wear Personal Protective Equipment, including face masks.
  • Change gloves frequently after handling evidence.
  • Do not touch areas on evidence that may be sampled for DNA.
  • Collect elimination samples from those who have been in contact with the evidence or scene.
  • Use disposable fingerprint brushes and powder.
  • Clean each crime scene tool coming into contact with evidence."

Just how valuable is Touch DNA evidence? Check out this video detailing how Touch DNA has helped Florida’s St. Petersburg Police Department solve nearly 40% of all burglary cases since the technology became available in 2010. Here are just a few applications for Touch DNA. 

Screen_Shot_2014-09-03_at_5.27.58_PMImage credit: Ryan, Suzanna, MS. Touch DNA.

If you would like more information on using our swabs for crime scene investigation or about how to collect a Touch DNA sample, please don’t hesitate to contact one of our knowledgeable product specialists today. 



Ryan, Suzanna, MS. Touch DNA. What is it? Where is it? How much can be found? And, how can it impact my case? A question and answer guide to all things touch DNA. Ryan Forensic DNA Consulting. Retreived September 3, 2014 from:

Collecting Touch DNA Evidence. Forensic Magazine. Retreived September 3, 2014 from:

Touch DNA: From the Crime Scene to the Crime Laboratory. Forensic Magazine. Retrieved September 3, 2014 from:

Touch DNA. DNA Forensics. Retrieved September 3, 2014 from:

Topics: Forensics

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