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

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.

Residual DNA on examination tools following use

Recent studies observing the transfer of DNA via examination tools used within forensic laboratories (scissors, forceps and gloves) have highlighted the contamination risk of such implements if protocols following their use and replacement are not adhered to. Whilst these previous studies focus primarily on the transfer of biological substances to a substrate via high-risk vectors, this investigation considers the proportion of DNA that remains on the high-risk vectors following contact with the substrate. Dried blood or touch DNA was deposited on cotton or glass substrates to create mock exhibits. Following primary contact with the deposit, the vector similarly contacted a secondary DNA-free substrate. Combinations of singular and multiple contacts were applied. Immediately following contact with the secondary substrate, the vector was sampled in order to determine the proportion of DNA-containing material remaining on the vectors following contacts. Residual DNA was detected on the vectors in most instances, with the amount retained influenced by the vector, substrates and biological substance applied. The results demonstrate the potential for inter- and intra-exhibit contamination through further contacts.