While touch DNA has become a much requested and successful test for DNA laboratories to perform, we must remember its limitations and be aware of the factors which may affect the results.While touch DNA has become a much requested and successful test for DNA laboratories to perform, we must remember its limitations and be aware of the factors which may affect the results.

When Edmond Locard proposed his Exchange Principle, he could not have imagined the implication it would have on forensic science today. Indeed, in the field of DNA testing, does “Every contact leave a trace” as Locard stated?

Initially in forensic serology, the “trace” left behind was a bloodstain containing antigens and polymorphic enzymes. Blood was visible in most cases for testing, but the end result was not always very discriminating. When Restriction Fragment Length Polymorphism (RFLP) DNA profiling was developed in the early 1990s, forensic serologists were excited to be part of a new generation of testing which was now being conducted at the molecular level. This early DNA testing required a relatively large amount of DNA for the development of a DNA profile, and the bloodstain left behind was still visible or detectable using presumptive tests.

By the late 1990s RFLP testing was being replaced by the Polymerase Chain Reaction (PCR) based short tandem repeat (STR) DNA typing method. A new era of trace DNA detection had begun. The quantity of DNA required for analysis had been dramatically reduced. What was once a visible bloodstain had now become a speck of blood barely visible to the naked eye. As STR testing continued into the new millennium, DNA profiles were beginning to be developed from evidence where there was seemingly no bloodstain or other body fluid present. This trace or “touch” DNA, as it is now referred to, led to many questions. What had changed? How can we test evidence for skin cells? Will the results be meaningful or probative to the case? How do we process a crime scene and collect evidence? How do we address contamination? Is touch DNA really a different technology?

Touch DNA
Epithelial or touch DNA evidence can be defined as evidence with no visible staining that would likely contain DNA resulting from the transfer of epithelial cells from the skin to an object. Can simply touching an object leave skin cells? It has been stated in publications that forensic scientists can obtain a DNA profile from as few as five to six cells. However, just because a surface is touched and a few skin cells are left behind does not guarantee a meaningful DNA profile can be obtained. Detecting and obtaining an interpretable DNA profile are two different concepts.

The touch DNA testing performed by most public crime laboratories utilizes the same STR typing method performed by DNA analysts for the last 10–15 years. It is not a new technology. There are alternate methods of detecting DNA from a smaller number of cells ranging from 15–30 diploid cells. This is known as Low Copy Number (LCN) testing and utilizes “enhanced” techniques for obtaining DNA profiles. The experts and courts have had disagreements on its validation, interpretation, use, and acceptance. Typically, STR typing is known to require approximately 70–150 cells to produce a DNA profile.


Why are DNA labs receiving more touch DNA requests?
Crime laboratories are receiving more property crime cases than ever before. The Combined DNA Index System (CODIS) has produced a large percentage of hits obtained from property crime cases and these profiles have been connected to other types of offenses as well. Violent crime cases with no blood or semen are being submitted to DNA labs in the hope that touch DNA testing may provide investigative leads. Cold cases where there may not have been body fluids left behind or the samples were too degraded are now being resubmitted to DNA labs.

Why are labs seeing more success with touch DNA cases?
Evidence collectors have more training in recognizing suitable items potentially handled by a suspect and how to collect touch evidence. DNA typing kits have improved in sensitivity and the number of loci detected, and instrumentation has improved in the areas of extraction, quantitation, and instrumental analysis.

What makes it likely that a suspect may leave his touch DNA on an item?
Some suspects are known to shed cells more than others. It is known that we shed approximately 400,000 cells daily.1 Does this mean suspects are like the Peanuts character, Pigpen, walking around leaving a cloud of cells capable of producing several thousand DNA profiles? The clothing worn by suspects reduces the number of cells that can slough off and deposit themselves on a surface capable of producing DNA results. Suspects are nervous and tend to perspire more during the crime, leaving more cells behind. A suspect may touch his face or head and transfer a large number of cells to his hands or gloves. Also, the rougher the surface texture, the higher the probability of abrading skin cells. Stronger contact pressure applied to the item also increases that probability.

Processing the Touch DNA Crime Scene
Is the crime scene technician wearing the appropriate personal protective equipment (PPE)? I sometimes refer to the image of a television crime scene investigator and ask my students, “What is wrong with this picture?” The image shows the investigator wearing latex gloves, but no hat, no mask, a short sleeved shirt, and dusting an item for prints. Epithelial cells from the head, arms, mouth, and nose may be left on crime scene evidence by the investigator. As far as dusting for prints is concerned, are the brush and powder disposable so as not to transfer cells from other items or even a previous crime scene?

Contamination is the unintentional introduction of outside DNA into a crime scene or laboratory sample. Contaminant DNA may appear as background DNA, the major or minor profile within a mixture, a single source DNA profile, or all of the above. When can this occur? Before the commission of the crime, after the crime and before the crime scene is discovered/secured, during the crime scene investigation, and within the crime laboratory or DNA laboratory. (Figure 1) In today’s world of touch DNA, a crime scene has to be approached in a way to minimize contamination since one cannot see or test for touch DNA.

Figure 1: Potential Sources of Epithelial Contamination

Figure 1: Potential Sources of Epithelial Contamination

Common rules to help minimize contamination at the crime scene:

  • Limit access to the crime scene or evidence
  • Do not talk over the evidence
  • PPE, including 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

Communication with the laboratory
A detailed request/submission form is necessary to alert laboratory personnel prior to analysis that potential touch DNA evidence may be present. Indicate if the suspect is a stranger or has been in contact with the scene. Note on the request if any of the touch exhibits will be shared with another section of the laboratory. This alerts other sections to take the appropriate measures to protect the touch DNA evidence. Provide the appropriate elimination standards whether from law enforcement, crime scene personnel, or individuals known to be in contact with the victim or scene. Follow up a touch DNA request with a phone call. Crime scene investigators should check with their forensic laboratory as it may have an evidence policy specifically for touch DNA evidence acceptance and examination.

The DNA Lab and Contamination
Once evidence is received into the DNA laboratory, how is contamination minimized and what are the potential sources of epithelial cells? The Quality Assurance Standards for Forensic DNA Testing Laboratories addresses DNA contamination in Standard 9.7. In the discussion section it states, “A laboratory shall have and follow a documented policy for detecting and controlling contamination. This policy should include the procedures used by a laboratory for monitoring, decontaminating, and detecting contamination. In addition, a laboratory shall have and follow policies and/or procedures for interpreting data potentially affected by contamination.”2

In addition to lab coats and masks, how is contamination detected, controlled, and monitored? Some examples which may be employed are:

  • Access to the laboratory shall be controlled and limited to prevent access by unauthorized personnel.
  • Separation of the work areas for evidence examination, DNA extraction, and DNA amplification and typing.
  • Exhibits are examined at different locations and/or times.
  • Negative controls are being used from evidence examination through extraction and amplification.
  • Contamination logs monitor introduction of exogenous DNA and identify sources.
  • Elimination profiles of past and present DNA analysts or support staff are maintained.

The European Network of Forensic Science Institutes (ENFSI), the FBI Laboratory’s Scientific Working Group on DNA Analysis Methods (SWGDAM) composed of U.S. and Canadian members, and the Australian Biology Specialist Advisory Group (BSAG) published a joint position statement advocating that forensic DNA laboratories maintain an elimination database for screening DNA results as appropriate.3


Contamination Events
In some recent cases both in the U.S. and the UK, there have been DNA profiles developed from crime scene evidence where the profiles matched a crime scene technician or DNA analyst and the information had been erroneously reported as a possible suspect profile. This can be avoided as long as we have the relevant DNA elimination databases. In one particular example, staff profiles from DNA analysts were not being maintained. An unknown profile was obtained and given to detectives as an investigative lead. It was later discovered the profile belonged to a DNA analyst. In a recent request for a new trial based on skin cells found on evidence not matching the defendant’s DNA, the Maine Judicial Supreme court ruled that there were other plausible sources for the male DNA, including detectives at the crime scene and forensic investigators who later handled the evidence. Later analysis of the DNA showed that the traces of male DNA could have come from a fingerprint brush the latent print analyst had used in other cases.4

What are some of the problems DNA analysts encounter with touch DNA cases?

  • No presumptive tests
  • Partial profiles yielding low statistics
  • Complex Mixtures: A mixed sample may contain background DNA, crime-related DNA, and post-crime contamination, and it may be difficult to identify the relevant profile. • Increased chance of contamination
  • The profile may not be sufficient to enter into CODIS
  • A late touch DNA request for examination of shared evidence
  • Re-examination of cold cases which may not have been collected, stored, or examined with trace DNA detection sensitivities in mind.
  • Touch DNA does not tell you “when” or “how” DNA was deposited
  • Background DNA obtained from clothing which was handled by someone else or by the manufacturer
  • A bloodstain containing epithelial cells from another source

Successful Touch DNA Results
In my own experience, I have seen touch DNA results obtained from some unusual sources. These included DNA profiles developed from a pocket lining, grips from weapons, asphalt, face epithelials transferred to clothing, utensils, shoestrings, and a victim’s oral swab which contained DNA from the suspect’s tongue. On one occasion, I swabbed my own hand after handshakes at a social function to determine the presence of other epithelial cells. The laboratory was able to obtain a mixture of my DNA as well as two other individuals.

While touch DNA has become a much requested and successful test for DNA laboratories to perform, we must remember its limitations and be aware of the factors which may affect the results. These include:

  • Was the amount of DNA adequate for meaningful interpretation?
  • Was contamination minimized and accounted for?
  • Did uninvolved individuals have access to the scene or victim?
  • Was it likely that a suspect could have deposited epithelial cells on the surface of the evidence?

If every contact leaves a trace, as Locard stated, then we must be prepared to answer these questions.


  1. Wickenheiser, RA. Trace DNA: A review, Discussion of Theory, and Application of the Transfer of Trace Quantities of DNA Through Skin Contact. J. Forensic Sci. 47 (3) (2002) 442-450.
  2. Quality Assurance Standards for Forensic DNA Testing Laboratories. September 1, 2011
  3. Butler, John M. Advanced Topics in Forensic DNA Typing: Methodology. Walham: Academic Press, 2012
  4. Christopher Cousins. Bangor Daily News. Maine Supreme Court Rejects DNA-based Murder Appeal 1-24-13.

Joe Minor, M.S. is a forensic DNA consultant and instructor in Forensic Science for Cumberland University and Austin Peay State University. He is a retired Special Agent-Forensic Scientist Supervisor/DNA Technical Manager and violent crime response team member from the Tennessee Bureau of Investigation.