DNA transfer by examination tools – a risk for forensic casework?

https://doi.org/10.1016/j.fsigen.2015.02.004Get rights and content

Highlights

  • The contamination risk of tools used during exhibit examination was evaluated.

  • Scissors, forceps and gloves are a contamination risk, they readily transfer DNA.

  • Transfer detectability is dependent on the substrates and biological sources involved.

  • Contaminating alleles may affect the probative value of DNA profiles in casework.

Abstract

The introduction of profiling systems with increased sensitivity has led to a concurrent increase in the risk of detecting contaminating DNA in forensic casework. To evaluate the contamination risk of tools used during exhibit examination we have assessed the occurrence and level of DNA transferred between mock casework exhibits, comprised of cotton or glass substrates, and high-risk vectors (scissors, forceps, and gloves). The subsequent impact of such transfer in the profiling of a target sample was also investigated. Dried blood or touch DNA, deposited on the primary substrate, was transferred via the vector to the secondary substrate, which was either DNA-free or contained a target sample (dried blood or touch DNA). Pairwise combinations of both heavy and light contact were applied by each vector in order to simulate various levels of contamination.

The transfer of dried blood to DNA-free cotton was observed for all vectors and transfer scenarios, with transfer substantially lower when glass was the substrate. Overall touch DNA transferred less efficiently, with significantly lower transfer rates than blood when transferred to DNA-free cotton; the greatest transfer of touch DNA occurred between cotton and glass substrates. In the presence of a target sample, the detectability of transferred DNA decreased due to the presence of background DNA. Transfer had no impact on the detectability of the target profile, however, in casework scenarios where the suspect profiles are not known, profile interpretation becomes complicated by the addition of contaminating alleles and the probative value of the evidence may be affected. The results of this study reiterate the need for examiners to adhere to stringent laboratory cleaning protocols, particularly in the interest of contamination minimisation, and to reduce the handling of items to prevent intra-item transfer.

Introduction

The presence, accumulation and transfer of extraneous DNA within a forensic laboratory is a fundamental issue as contaminating DNA can be detrimental to profile analysis and subsequent investigative outcomes. Although the existence of background DNA in a laboratory setting is deemed to be low [1], [2], its mere presence provides the opportunity for DNA transfer and the potential to contaminate evidentiary samples. Furthermore, the detection of previously undetected trace elements of DNA has increased due to the recent introduction of more sensitive DNA typing systems [3]. In such instances, the level of DNA in the targeted sample influences the detectability of any contaminating DNA, with contamination more likely to be detected in cases where the target sample is present in only trace quantities.

Whilst some understanding of key variables affecting transfer has been acquired, especially in relation to biological sample type, sample moisture, substrate type and manner of contact [4], [5], [6], [7], it is clear that a greater understanding is required to assist evaluations of potential scenarios where transfer plays a role [8]. Laboratory-based simulations reconstructing crime-scene scenarios are useful in the assessment of DNA transfer and the probability of attaining positive DNA profiles in such instances [9], however very little has been done to investigate transfer within the laboratory and its potential to contaminate evidentiary samples.

Contamination within the laboratory may take place as a result of unprotected speaking [10], through the presence of DNA on unused laboratory gloves [11], and ineffective cleaning procedures [3], [12], [13]. A recent preliminary study observed the transfer of dried blood from and to cotton via tools used during examination (scissors, forceps and gloves), and indicated that if these instruments are not adequately cleaned, they pose a high contamination risk [14]. Here we extend this study by repeating preliminary tests (transfer of dried blood to DNA-free secondary substrates) to improve the assessment of the level of risk, and by performing additional tests that explore the transfer of touch DNA, a greater variety of heavy and light transfer scenarios, an additional substrate type (hard/non-porous glass), and transfer to non-DNA free secondary substrates, deposited with blood or touch DNA, and the effect this has on the profiling of a target sample.

Section snippets

Deposition of blood

Venous blood (25 μL) from Person A (male) or Person B (female) was deposited on cotton drill (Lincraft, Australia), as per Szkuta et al. [14]. An additional 25 μL of blood from Person A was deposited onto 1.5 × 1.5 cm areas of glass slides (7.6 × 2.6 cm, MENZEL, Germany). Blood deposits were air-dried at room temperature (18–24 h). The same donors were used throughout the course of the experiments. Donors were not analysts within the laboratory where the experiments were carried out.

Deposition of touch DNA

2–12 h prior to

Transfer of primary deposit from cotton to DNA-free cotton (experiment 1)

In general, transfer was significantly higher for dried blood compared to touch DNA (P < 0.001), with transfer observed for all vectors in all heavy and light transfer scenarios explored. Gloves facilitated the greatest amount of transfer (87–99%), with little impact of contact level on transfer amounts. In contrast, transfer was low for forceps in all scenarios, and particularly when contact was light (Fig. 2).

On average the transfer of touch DNA was minimal with 53% fewer alleles transferred

Transfer of primary deposit from cotton to DNA-free cotton (experiment 1)

Experimental findings from the transfer of dried blood via scissors, forceps and gloves are concordant with the results of Szkuta et al. [14], although direct comparisons can only be made regarding heavy/heavy and light/light transfer scenarios. Whilst the previous study demonstrated the greatest transfer of dried blood via scissors, in the current study, gloves facilitated the greatest transfer with an average of 21% more alleles compared to scissors and 42% more than forceps. Variation in the

Conclusion

Results from this study demonstrate that DNA-containing material can be transferred from exhibit to exhibit by scissors, forceps and gloves. It is clear they pose a significant contamination risk if not DNA-free before contact is made with the targeted sample during exhibit examination, although the contamination risk of this source is reduced/eliminated if the deposit area is not touched. The reuse of instruments and further contact with other areas of an exhibit, could potentially relocate

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