Emily C. Lennert
trace DNA, touch DNA, DNA, PCR, STR, profiling, clothing, contact
Hess, S.; Haas, C. Recovery of trace DNA on clothing: A comparison of mini-tape lifting and three other forensic evidence collection techniques. Journal of Forensic Sciences. 2017, 62(1), 187-191.
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, often referred to as touch DNA, is DNA recovered from epithelial, i.e. skin, cells that are deposited upon contact with an item. Trace DNA may be left on clothing, weapons, and other objects, and the transfer of trace DNA is instantaneous upon contact. When recovering trace DNA, the nature of the object from which it is obtained may influence the recovery of DNA; epithelial cells have been shown to adhere best to porous surfaces rather than nonporous surfaces. Regarding clothing, it is expected that epithelial cells adhere better to natural fibers compared to synthetic fibers. Other factors may also affect DNA recovery, e.g. the presence of polymerase chain reaction (PCR) inhibitors such as certain dyes. Overall, the success of DNA recovery is largely dependent on the collection method. Swabbing, either wet or dry, and scraping are common methods of DNA evidence collection from clothing. More recently, adhesive mini-tapes have seen use as a method for the recovery of DNA. In this study, mini-tape lifting, wet swabbing, dry swabbing, and scraping were compared for trace DNA collection from six different types of clothing.
Six types of clothing were used in this study: blue jeans, raincoats, dark synthetic fabrics, bright synthetic fabrics, dark natural fabrics, and bright natural fabrics. A description of the clothing can be found in Table 1 within the study. Blue jeans were handled as a separate category due to the frequency of which the material is encountered in casework. The raincoat material was a synthetic fabric coated with PVC, making the material nonporous; all other materials were porous. Clothing was worn by three volunteers, serving as the “victims”, then washed twice to remove as much of the victim’s DNA as possible. Clothing was handled to prevent transfer of DNA prior to the application of “perpetrator” DNA. Three new volunteer, 2 male and 1 female, served as the “perpetrators”. Each perpetrator grabbed an area of the clothing for five seconds, in triplicate, on four places on each of the pieces of clothing with the exception of raincoats, which were only grabbed in three places. Scraping is not routinely used on nonporous materials, so only three sample areas were necessary for raincoats. Contact zones were circled to mark areas for DNA recovery.
Samples were collected from each garment using each sampling technique: mini-tape lifting, wet swabbing, dry swabbing, and scraping. Swabbing was performed by rubbing a dry or wet cotton swab with moderate pressure and rotation on the contact area; wet swabs were moistened with sterile water. Mini-tape lifts were collected by using adhesive tape, which was transferred to a sterile 1.5 mL tube after sampling. Scraping was conducted with a sterile scalpel, and collected material was placed in a sterile 1.5 mL tube. Following collection, DNA was extracted from each sample by conventional Chelex extraction using standard protocol. DNA was then amplified by PCR, and PCR products were separated and detected to generate short tandem repeat (STR) profiles.
Perpetrators 1 and 2 (the male volunteers) were considered good shedders of epithelial cells, with 90% and 82.6% of alleles detected, respectively. Perpetrator 3 was considered a poor shedder, with only 14.2% of alleles detected. Therefore, samples were divided into two categories: good and poor shedders. Overall, mini-tape lifting and scraping produced samples with the highest DNA recovery: 67% and 73% alleles detected, respectively. Dry and wet swabbing produced lower DNA yield, with 52% and 58% alleles detected, respectively. When examined by group, i.e. good shedder or poor shedder, mini-tape lifting produced 92% of alleles detected for good shedders and 18% of alleles detected for poor shedders. By the scraping method, 93% of alleles were detected for good shedders and 32% of alleles were detected for poor shedders. Dry swabs produced 76% of alleles detected for good shedders and 2% detected for poor shedders, while wet swabs produced 82% of alleles detected for good shedders and 9% of alleles detected for poor shedders. While this trend was true for most materials, raincoats showed highest DNA recovery for mini-tape lifting (65%) and dry swabbing (60%). Scraping was not performed on raincoats, and wet swabbing produced the lowest recovery (51%).
The authors stated that, regardless of sampling method, better results were obtained from dark natural materials, including blue jeans, than from synthetic materials. When examining synthetic materials, it was observed that good shedders produced better results on bright synthetics compared to dark synthetics, while poor shedders produced better results on dark synthetics compared to bright synthetics. Good shedders had worst recoveries on dark synthetic materials, while poor shedders had worst recoveries on raincoats.
Mixed DNA profiles were obtained in 40% of samples. In several cases, these DNA mixtures could be attributed to the victim and perpetrator, despite the attempt to wash away the victim’s DNA. In two samples, the unknown DNA was found to have originated from a research staff member. One sample was attributed to single tube contamination. Finally, one STR profile appeared in several samples from two specific articles of clothing and could not be attributed to any known person.
- Regardless of fabric type, mini-tape lifting was among the best methods for DNA recovery in this study.
- Whether the DNA donor was a good or poor shedder had an impact on which material DNA was best recovered from.
- DNA was recovered best from natural fibers compared to synthetic fibers.
Examiners must understand the best collection method for trace DNA to recover the most DNA from a piece of evidence.
Mini-tape lifting may provide the best overall recovery of DNA from a variety of clothing materials.