DNA Sampling

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DNA Sampling


The use of new techniques in the analysis of DNA can serve to provide information connecting suspects to crimes where, in the past, were considered impossible to solve. The techniques used are very much like the standard approach in the collection of DNA evidence. However, the sample size required is less than typically needed. This technique is referred to as touch (or trace) DNA collection. Though there are limitations in the scope of this technique, the ability to solve some crimes has been enhanced greatly and results in a potent analysis tool for forensic scientists.

Keywords: Touch DNA, DNA collection, forensic science, sample collection techniques

DNA Sampling

By the late 1990s Restriction Fragment Length Polymorphism (RFLP) testing was being used less and substituted by the Polymerase Chain Reaction (PCR) based short tandem repeat (STR) DNA typing technique. Though the RFLP testing was only about a decade old, technology had ushered in a new period of trace DNA detection. Touch DNA analysis has developed into an essential part of a forensic laboratory’s assigned work and a vital instrument for investigators. For that reason, there has been substantial research conducted in order to examine the characteristics of touch DNA and the best technique to improve its collection, augmentation and explanation (Oorschot, Ballantyne, & Mitchell, 2010). No longer was it necessary to have as large of a quantity of DNA for analysis. STR testing had reduced the necessary sample size of a bloodstain to a mere speck to achieve a DNA profile. The seemingly absent sample size, consisting of an incredibly inadequate amount of blood or body fluid, would become what is referred to as trace or touch DNA (Minor,2013). In epithelial (or touch) DNA evidence there appears to be no evidence present because there is no visible staining occurring that would suggest a transfer of epithelial cells from the skin to an object had taken place. “It has been stated in publications that forensic scientists can obtain a DNA profile from as few as five to six cells (Minor,2013, p. 1).

Touch DNA

Touch DNA, known as, low-level DNA, can even be obtained from a victim’s skin or bruises at locations where the victim was handled violently. Low-level DNA samples may prove to be valuable when examining evidence where there is difficulty in retrieving fingerprints. Textured surfaces on automobile dashboards or gun handles can create complications to obtaining good fingerprint samples (National Forensic Science Technology Center, 2012).

Touch DNA testing uses the identical STR and PCR technology utilized to test more conventional sources of DNA (blood, saliva, semen, and other bodily fluids) to analyze recovered epithelial cells. The dissimilarity between conventional DNA testing (the analysis of bodily fluids) and touch DNA testing is the substance from which the DNA is collected, not the technique by which the DNA sample is evaluated. Even though touch DNA testing uses the same PCR-STR mechanics that is used to test other broadly accepted sources of DNA, it has proven to be unsuccessful in earning the same approval as its bodily fluid predecessors (Kawecki, 2013).

Touch Sample Collection

The initial step in the collection of touch samples is to identify the areas expected to have them, although they are not readily identifiable, by and large. Typically, fingerprinting agents are utilized to identify those areas touched on exhibits; however, many exhibits are swabbed or tape lifted based upon an assumption about where the DNA-containing material is thought to be located. The ideal situation would be the use of non-invasive detection systems such as Polilight; although not used on a widespread scale. Use of a Polilight System, which uses a high-intensity, light filtration system to detect body fluids and fingerprints, may not be available due to cost or perceived impracticality within a specific scope of work. Touched surfaces that have been exposed using fingerprinting methodologies are typically those surfaces on which fingerprints are pursued as the priority, rather than surfaces where DNA will be sampled. Even though many fingerprinting techniques do not negatively affect the quality of retrieved DNA, some techniques can do so and others potentially lessen the quantity of retrieved DNA. When considering the subsequent uses of the retrieved fingerprint, extra emphasis should be given to the influence of the fingerprinting methodology used on downstream DNA retrieval and value. (Oorschot, Ballantyne, & Mitchell, 2010).

Sample Collection Techniques

The characteristic sample collection entails the use of touch DNA and cotton swabs. Touch DNA integrates the use of skin cells that an individual has transferred onto objects he or she has touched. These transferred cells can then be tested using DNA analysis procedures. Touch DNA has been observed to offer a greater amount of forensic evidence than fingerprinting. Noteworthy is that evaluations using touch DNA does not automatically indicate that more individualized identifications are produced. Collecting sample cells can also be achieved with the use of a cotton swab moistened with sterile water. The moistened tip of the cotton swab is dabbed or wiped across the specimen region and then applying a dry sterile swab to collect the water and cells (Romeika &Yan, 2013). “The amount of DNA transferred to a substrate during handling was found to be independent of handling time, dependent on the individual handler and dependent on the handled substrate” (Daly,Murphy,& McDermott,2011, p.1).

Further progress in the techniques for identifying the biological source of touch samples (beyond the realm of fingerprints) on targeted surfaces, and their function in the course of forensic investigations, should help to enhance sample collection. Swabbing an assumed touch sample area, that is not as large as the actual deposition area will indicate that a portion of the sample remains uncollected. Swabbing an area larger than the actual area of deposit could signify that sample is distributed over a more extensive area and that less is collected. Both of these approaches have the prospective to result in an inaccurate view of the location the actual sample was found. It is, for that reason, not only essential to be aware of the exact location of the substance being targeted but also to collect from the area properly (Oorschot, Ballantyne, & Mitchell, 2010).

Limitations of Touch DNA

Recognizing that touch DNA sampling techniques and the required following DNA processing procedures are very susceptible to contamination, there is greater potential of detecting contamination from forensic scientist or law enforcement personnel interaction. The contamination can even take place when the proper personal protective equipment (PPE) has been worn. It may be obligatory to acquire elimination samples from vital personnel in the case where foreign DNA profiles are collected that cannot be ascribed to a suspect or the victim. An increased chance exists of acquiring mixed DNA profiles that contain DNA from individuals potentially coming into contact with the victim/evidence item proximate to the time of the crime. Those contributing to these combinations could include the victim’s spouse or children. In that case, elimination samples should be collected from each of these individuals (Williamson,2012).


The forensic investigator could also be faced with the difficulty of determining what is meant if unexplainable DNA is collected. An example would be in the case a foreign male profile resulting from a touch DNA sample might be acquired from evidence linked to a female victim. If the male DNA profile fails to match the suspect in question, the forensic investigator would be prudent to consider its significance to the case. The foreign profile could be from the actual perpetrator and the original suspect may very well be innocent. Another possibility is that the DNA profile is from unplanned transfer from crime scene personnel, whether the first responders, crime scene equipment, or laboratory analysts.

Some evidential items are not compulsory for the gathering of touch DNA samples. These items consist of those that are severely damaged having been exposed to severe environmental, have been laundered, or have been heavily drenched in the victim’s body fluids. It is also possible the items are likely to have been touched by numerous people. Examples of these items would be shopping carts, vending machines, public pay phones, or store counters as they are typically compromised sources for probative or interpretable touch DNA profiles (Williamson,2012).


“The key to obtaining successful touch DNA results depends on recognizing items which may be suitable for touch DNA analysis, proper collection at the crime scene, and the application of a sampling technique that will recover the highest number of skin cells,” (Williamson, 2012, p. 4). Through enhancement in the methods used to sample DNA combined with progressively more sensitive DNA testing methods, and through persistent education of the criminal justice community regarding the testing possibilities, touch DNA is providing forensic scientists information in cases which were at one time considered unsolvable (Williamson,2012).


Daly,D.J., Murphy,C., & McDermott,S.D. (2011). The transfer of touch DNA from hand to glass, fabric, and wood. Forensic Science International: Genetics, 702(1), 1. doi:10.1016/j.fsigen.2010.12.016

Kawecki,V. (2013). Can’t Touch This? Making a Place for Touch DNA in Post-Conviction DNA Testing Statutes. Catholic University Law Review, 62(3). Retrieved from http://scholarship.law.edu/cgi/viewcontent.cgi?article=1054&context=lawreview

Minor,J. (2013). Touch DNA: From the Crime Scene to the Crime Laboratory. RetrievedMarch14, 2015, from http://www.forensicmag.com/articles/2013/04/touch-dna-crime-scene-crime-laboratory

National Forensic Science Technology Center. (2012). A simplified guide to DNA evidence. Retrieved from https://www.ncjrs.gov/pdffiles1/nij/grants/220692.pdf

Oorschot,R.A., Ballantyne,K.N., & Mitchell,R.J. (2010). Forensic trace DNA: a review. Investigative Genetics, 1(14). doi:10.1186/2041-2223-1-14

Romeika, J.M., &Yan, F. (2013). Recent Advances in Forensic DNA Analysis. J Forensic Res S12: 001. doi: 10.4172/2157-7145.S12-001

Williamson,A.L. (2012). Touch DNA: Forensic collection and application to investigations. Retrieved from http://www.acsr.org/wp-content/uploads/2012/01/Williamson.pdf