Brief CommunicationDouble swab technique for collecting touched evidence
Introduction
Evidentiary touched items are seized and submitted to crime laboratories for DNA analysis since van Oorschot and Jones have reported that DNA profiles could be obtained from handled objects [1]. DNA recovered from these objects was thought to have originated from epithelial cells [2]. A number of studies have examined the primary and secondary transfer of DNA by using single dampened cotton swabs for collecting DNA samples from touched objects [2], [3], [4]. The current sensitivity of the detection instruments and PCR–STR techniques has been able to produce full DNA profiles at or below 100 pg of purified DNA [5]. Full DNA profiles from single cells could be obtained using six forensic STR markers [6].
Double swab technique was originally advocated by Sweet et al. for recovery of saliva from skin. A wet cotton swab and a second dry cotton swab are applied onto the same surface of interest in the double swab technique. The use of the double swab technique improved the recovery of saliva compared to the use of the classical stain recovery technique [7]. Furthermore, the double swab technique was also used to retrieve trace level of DNA in the study of the primary and secondary DNA transfer [8], [9], [10], [11]. Even though the double swab technique has been applied in these studies, the usefulness and advantages of using the technique to recover touched DNA samples have not been discussed.
In this study, the classical stain recovery technique and the double swab technique for recovering DNA on touched objects were evaluated. Experiments were carried out to examine the efficiency in the use of the double swab technique to obtain sufficient DNA to yield reportable DNA profiles. The use of the double swab technique for touched evidence collection at crime scenes was also discussed.
Section snippets
Samples and swabbing
Skin cells sloughed off during contact were collected by a wet swab first and then by a dry swab following the double swab technique [7] on the surfaces of various articles found in this laboratory (Table 1). The wet swabs were prepared by dipping the sterile cotton bud swab (Sterile swabs from Medical Wire & Equipment, Corsham, Wiltshire, England) into sterile water. The target surface was swabbed by a wet swab and then a dry swab with the same swabbing protocol. The surface was swabbed for
Results
Table 1 summarizes the DNA results obtained from the wet and the dry swabs. Positive DNA typing was found in 16 out of 20 wet swabs and 12 out of 20 dry swabs. Sixty percent of the dry swabs consisted of sufficient amount of DNA for producing a DNA profile (at least one allele was detected). Of the 12 dry swabs showing positive DNA profiling results, 2 of the corresponding wet swabs (Areas 13, 18, Table 1) gave negative DNA profiling results. It indicated that the single dampened swab might not
Discussion
No standard contact surface was defined in the experiments carried out in this study. It might be impossible to deposit a controlled number of epithelial cells from a known source onto a surface in a way similar to the natural shedding of cells during contact. It was observed that people deposited different amounts of epithelial cells through contact in different times and the amount of cells transferred from each contact of the same individual could vary widely [10]. The amount of DNA
Conclusions
The use of a wet and dry double swab technique for recovery of touched evidence improves the DNA profiling results and is useful in collecting the evidence at the crime scenes. This study shows that (1) the single wet swab may not recover epithelial cells present on the surface efficiently; (2) DNA recovered by the second swabs alone can produce DNA profiles. Since detectable amount of DNA could be recovered by the second dry swab from the moisture left by the first wet swab, the DNA profiling
Acknowledgements
The authors thank the Government Chemist, Dr. Ting T.L., the Assistant Government Chemist, Mr. Leung S.C. for their support of this work.
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