Elsevier

Legal Medicine

Volume 9, Issue 4, July 2007, Pages 181-184
Legal Medicine

Brief Communication
Double swab technique for collecting touched evidence

https://doi.org/10.1016/j.legalmed.2006.12.003Get rights and content

Abstract

Touched evidence is often submitted to forensic laboratories for DNA analysis. Classical stain recovery technique, involving one wet cotton swab, is commonly used for recovering the touched evidence. Double swab technique, using a wet cotton swab followed by a dry cotton swab, was compared with the classical technique for recovering the touched evidence. The wet cotton swabs and the dry cotton swabs were individually extracted. DNA extracts were quantified and amplified at 15 polymorphic loci. DNA recovered in some of the second dry swabs contained sufficient amount of DNA to yield a DNA profile. This study shows that the double swab technique improves the quality of the resulting DNA profiles. The double swab technique for recovering touched evidence at crime scenes is recommended.

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.

References (11)

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