With modern technology, the simple act of picking up an object or touching a surface can lead to the identification and apprehension of a criminal. In the past few years, not only have the number of touch DNA evidence items being submitted to the lab for analysis skyrocketed, but the number of journal articles regarding touch DNA and DNA transfer (both primary and secondary) has also increased greatly. This article is intended to update the reader on the latest touch and transfer DNA research and attempts to answer some of the most common questions that are asked regarding the topic.
What is “touch” DNA?
First, a review of what touch DNA is and how it arrives on an object. Touch DNA is simply DNA that is transferred via skin cells when an object is handled or touched. The average human sheds roughly 400,000 skin cells per day;1 however, since it is known that the top-most layers of skin are basically “dead”, being keratinized and having lost their nuclei,2 where does the touch DNA come from? Kita, et al,2 performed experiments which showed that small amounts of fragmented DNA are present on the surface of the skin and they theorized that these fragments of DNA may be constantly sloughed off the keratinized cornified layer of skin and that sweat may also contain fragmented DNA. Later research verified that the presence of sweat helps to contribute to the DNA profile obtained from touch DNA samples.3 This research also showed that cell free nucleic acids, or CNAs, (basically free-floating DNA fragments not encapsulated in the cell nucleus) contribute greatly to the total amount of DNA present in a sample with CNAs being detected in the sweat of 80% of healthy individuals tested. It was also found that, along with CNAs, nucleated cells were present in sweat samples taken from volunteers. Interestingly, most DNA extraction methods do not utilize the portion of the sample where CNAs are found—the aqueous portion of the extract—and after centrifugation to collect the cellular material, the supernatant (containing the CNAs) is generally discarded.3
How much DNA is left behind when an object is touched?
One of the most common questions asked regarding touch DNA is, “how much DNA is expected to be transferred” given a certain set of circumstances. Unfortunately, this is a difficult question to answer as there are so many variables involved. However, we can use the information from various studies to provide an idea of how much DNA might be recovered from touched objects (Table 1).