Emily C. Lennert
touch, DNA, car-jacking, theft, steering wheel, double-swabbing, cotton swab, FTA paper, FTA card, tape lifting, profile, partial, PCR, STR
- Kirgiz, I. A.; Calloway, C. Increased recovery of touch DNA evidence using FTA paper compared to conventional collection methods. Journal of Forensic and Legal Medicine. 2017, 47, 9-15.
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.
When a person comes in contact with a surface, small amounts of skin cells may be shed and deposit on the surface. The DNA that can be recovered from these cells is referred to as touch DNA. Touch DNA analysis requires a minute amount of DNA and can be useful in forensic investigations. For example, in the event of a vehicle theft, touch DNA may be recovered from a steering wheel and used to link a suspect to the vehicle, therefore linking the suspect to the crime. When a small amount of DNA is recovered, such as in touch DNA, oftentimes a complete DNA profile cannot be obtained from the sample and a partial profile is obtained instead. Sampling methods can help to increase the amount of DNA recovered, which will in turn increase the chances of obtaining a complete DNA profile. The most common method of touch DNA collection is by cotton swabs, in a procedure known as double swabbing. In double swabbing, two cotton swabs are moistened and used to swab the area where touch DNA is suspected to have been deposited. Another method of collection is tape lifting, which uses a water soluble tape. To collect touch DNA, the tape is repeatedly applied to the area where touch DNA is suspected to be. In this study, the authors compare these conventional methods of touch DNA collection with a novel method, a scraping method using Whatman® FTA® paper cards, to determine the most effective method of touch DNA collection.
Citing the vehicle theft, i.e. “carjacking”, example, the authors chose to obtain samples from vehicle steering wheels. A total of 70 volunteers allowed their vehicles to be used in the study. Vehicles were selected based on the steering wheel meeting the selected criteria: a hard plastic composition, relatively clean, and no steering wheel cover. Each vehicle was driven for 2-60 min on the day of sample collection, after which samples were collected from each steering wheel. Reference DNA samples were collected from each driver using cotton cheek swabs. Samples were collected as follows: half were used to compare double swabbing to tape lifting, and half were used to compare double swabbing to FTA® paper scraping. FTA® paper scraping is carried out by wetting a piece of FTA® paper, and wiping, i.e. “scraping”, the surface to remove touch DNA. On 35 steering wheels, one side, i.e. right or left, of the steering wheel was sampled using the double swab technique while the other was sampled with the tape lifting method. For the remaining 35 steering wheels, one side of the steering wheel was sampled using the double swab technique and the other side was sampled using the FTA® paper scraping method. The side sampled for each method was alternated randomly to minimize any differences in touch DNA due to favored driving hand or potential differences in cell shedding between right and left hands.
Double swabbing was carried out with both swabs moistened with 3 drops of DI water: the area was swabbed by the first cotton swab, followed by the second. Tape lifting was carried out using a 6 cm piece of water soluble tape. FTA® paper scraping was carried out with a 3.2 x 3.9 cm rectangle of FTA® paper, wetted with 4 drops of DI water. After collection, DNA was extracted, i.e. removed, from each sample. DNA was extracted from cotton swabs for both steering wheel samples and reference samples according to manufacturer’s procedures for buccal swab extraction. Likewise, manufacturer’s procedures were followed to extract DNA from the FTA® paper. Tape samples were placed in a beaker of 60 ˚C DI water and agitated for 1 min to elute DNA. Following DNA extraction, each sample was processed following a standard DNA extraction procedure using QIAGEN® QIAmp DNA Kit, then 50 μL of nuclease-free water was added to each sample.
Samples were then analyzed by quantitative polymerase chain reaction (qPCR) to quantify the amount of DNA present in each sample. Each sample was quantified twice to allow for more accurate estimation of the quantity of DNA present. A total of 140 samples were obtained, 2 from each steering wheel and 70 steering wheels sampled. Of the 140 samples, 16 pairs (32 samples) were selected for further analysis: 8 pairs (16 samples) from the double swab/tape lift steering wheels and 8 pairs (16 samples) from the double swab/FTA® paper steering wheels. Short tandem repeat (STR) analysis was performed to allow for profiles of the DNA samples to be determined. The corresponding reference samples were also profiled. Each sample pair was then compared to determine how well a STR profile could be recovered by each sampling method.
Each of the 70 steering wheels yielded detectable amounts of touch DNA for each collection method. However, the yield of DNA varied depending on the sampling method. When comparing double swabbing to tape lifting, the average DNA yield of double swabbing was approximately two times higher, with an average yield of 16.1 ng, compared to tape lifting, which averaged 7.37 ng. However, double swabbing had a wide range of recovery, from very low to very high quantities, and the median yield for double swabbing and tape lifting were similar. Statistical analysis revealed no significant difference in DNA yield between double swabbing and tape lifting.
When comparing double swabbing to FTA® paper scraping, the average DNA yields were similar, averaging 5.22 ng for double swabbing and 5.33 for FTA® paper scraping. However, the median yield for double swabbing was approximately half, 2.26 ng, that of FTA® paper, 4.78 ng. Statistical analysis revealed a significant difference in DNA yield between double swabbing and FTA® paper scraping, with FTA® paper producing higher DNA yield compared to double swabbing.
DNA samples analyzed by STR analysis results indicated that 55% of double swabbing samples and 60% of tape samples would have produced partial or no STR profile due to low DNA yield, while the remaining 45% and 40%, respectively, produced complete profiles. Only 35% of FTA® paper samples produced DNA yields low enough to expect partial or no STR profiles, while the remaining 65% produced complete profiles. Of the 32 samples that were analyzed by STR, full profiles were obtained for 21 samples, partial but searchable profiles were obtained for 9 samples, and 2 tape lifting samples produced no STR profiles. Examination of the 8 pairs of double swab/tape lifting samples showed that double swabbing produced more complete profiles than tape lifting. Examination of the 8 pairs of double swab/FTA® paper samples showed that double swabbing produced slightly more complete profiles than FTA® paper, but the results were similar.
FTA® paper scraping was found to produce significantly higher DNA yield compared to conventional methods of touch DNA collection. Additionally, the profiles obtained from samples collected by FTA® paper scraping are similar to those obtained by double swabbing, a common conventional technique.
- FTA® paper was found to recover more touch DNA compared to conventional collection methods.
- Touch DNA recovered by FTA® paper produces profiles comparable to profiles produced from double-swabbing samples, a common conventional method.
- Tape lifting produced the lowest yield and most incomplete DNA profiles.
Touch DNA relies on a small amount of DNA to generate a full or searchable partial profile. Therefore, it is paramount that collection methods produce the highest yield possible to ensure the highest quality DNA profile possible.
- FTA® paper may be more effective than the more common, conventional double-swabbing collection method.
- Of the conventional methods, tape lifting is an inefficient option.