Emily C. Lennert
trace, DNA, STR, profile, sampling, swabbing, cutout
Dziak, R.; Peneder, A.; Buetter, A.; Hageman, C. Trace DNA sampling success from evidence items commonly encountered in forensic casework. Journal of Forensic Sciences. 2018, 63(3), 835-841.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
Trace DNA is a common form of forensic evidence. Challenges that are presented to laboratories include the selection of items to sample for DNA and the proper choice of sampling technique. Two common sampling techniques include swabbing and sample cutouts. In this study, optimal sampling strategies are suggested based on the success of obtaining short tandem repeat (STR) profiles from various casework and volunteer samples.
First, data was collected from casework to determine the success of STR profiling from common casework items containing trace DNA. Items included winter hats, baseball caps, fabric gloves, latex gloves, knives, firearms, fingernails, and more. Data from a total of 764 items was collected from case files dating 2005-2009 at the Biology Section of the Centre of Forensic Sciences, Ministry of Community Safety and Correctional Services (Ontario) in Toronto, Canada.
All casework samples had greater than a 10% success rate for the recovery of usable STR profiles, meaning that all casework sample types were potentially useful for evidence. Highest success rates were observed for baseball caps and winter hats, at 59% and 64%, respectively. These items were significantly more likely to produce usable STR profiles than all other items examined. Fabric gloves were significantly more likely to produce useful STR profiles than firearms, with a 33% success rate. Firearms, knives, footwear, fingernails, and duct tape were the least likely to produce usable STR profiles.
Volunteer samples were also collected. Table 1 within the study displays the 155 volunteer samples that were collected, along with the sampling protocol used for the item. For example, for bicycles, one swab was used to sample both handles of the bicycle. DNA was extracted and quantified. Samples with quantification values of 40 pg/L and above were amplified and STR profiles detected.
Volunteer samples all had success rates of greater than 18%, indicating that all items were potentially useful forms of evidence. Eyeglasses, with a success rate of 89%, were significantly more likely to provide useful STR profiles than all other items, with the exception of earphones, which had a success rate of 78%. Cutouts had significantly higher success rates than swabbing for sampling the inner headband area of winter hats, with 25% success for swabbing compared to 81% success for cutouts. Fabric gloves, baseball caps, and footwear yielded differences in success rates between swabbing and cutouts that were not statistically significant.
Sampling location was varied for fabric gloves and footwear, and was not found to produce significant differences in success rates. For eyeglasses, higher quantities of DNA were recovered when the entire frame was swabbed versus when the standard swabbing protocol was followed, however this was not found to be a significant difference. Standard protocol was the swabbing of the nosepiece and the portion of the arm that sits on the ear. All eyeglasses provided enough DNA for amplification when the entire frame was swabbed, however, two pairs did not provide enough DNA for amplification when the standard protocol was used. For latex gloves, higher DNA yield was observed when the fingertip area was swabbed compared to when the palm and hand area was swabbed, however, the difference was not determined to be statistically significant. Eight of eighteen gloves provided enough DNA for amplification when the palm and hand area was swabbed, whereas thirteen provided enough DNA for amplification when the fingertip area was swabbed. All gloves provided enough DNA for amplification if the entire inner surface of the glove was swabbed. Both eyeglasses and latex gloves had high variability between samples in the amount of DNA recovered.
Shirts were washed and worn by one individual for 24 hours, then sampled. The inner collar and armpit area were sampled by cutout and swabbing. A significant difference in success rate was observed between the collar swab samples and the armpit cutout. Additionally, compared to the armpit cutout, the collar cutout had significantly higher success rates. Shirts were also washed, worn by one individual for 24 hours, then worn by another individual immediately after for 24 hours. In these samples, the success rate dropped for collar swabs and were comparable to cutouts. Winter gloves that were habitually worn by volunteers were sampled by swabbing and cutouts. Multiple locations were sampled in the gloves, and differences in success rates for each location and sampling method were not found to be statistically significant. New winter gloves were then worn habitually over one month by one individual, then worn by another individual for one week. Most STR profiles that were recovered were mixtures of DNA from three or more individuals. Gloves were not washed prior to wearing, so the DNA may have originated from handling by other individuals, such as employees or customers, in the store prior to purchase. Major contributor varied between samples for each sampling technique. Most samples sampled by swabbing showed that the last wearer was the major contributor, whereas, for samples collected by cutout, the habitual wearer was most often the major contributor. Finally, socks were analyzed. The entire inside surface was swabbed or a cutout was taken from the heel area. Differences in success rate for location and sampling technique were not statistically significant.
Table 2 within the study summarizes the items with the highest success rates, as well as the recommended sampling strategies for each item. As a result of this study, the Centre of Forensic Sciences sampling protocol changed for some items. For example, the authors reported that the entire frame of eyeglasses is now being swabbed.
- All items analyzed in this study had potential for successful recovery of trace DNA.
- Eyeglasses and earphones provided the highest success rates for DNA recovery and STR profiles suitable for analysis.
- Table 2 within the study offered suggested sampling items and methods for the most likely recovery of usable STR profiles.
Proper item selection for sampling and sample collection technique may increase the recovery of trace DNA.
The results of this study may help to guide investigators regarding what samples to prioritize for DNA recovery and the most efficient way to process these samples.