Issues surrounding the impact of contamination on casework reporting guidelines were explored by Peter Gill and Amanda Kirkham in a recent article entitled 'Development of a simulation model to assess the impact of contamination in casework using STRs' (Gill and Kirkham 2004). Negative controls were used to predict the level of overall contamination in an operational DNA unit. However, because PCR contamination can be tube-specific (see above), negative controls run with a batch of samples cannot provide complete confidence that the associated batch of extracted casework material is contaminant-free
(Gill and Kirkham 2004). This study concludes that the most likely outcome of a contamination event is false exclusion because contaminating DNA material can be preferentially amplified over extremely low levels of original material present from the casework sample or may mask the perpetrator's profile in a resulting mixture.
While this contamination possibility might only rarely impact a careful forensic DNA laboratory, it can have potential significance on old cases under review including the Innocence Project (see D.N.A. Box 1.2). For example, if biological evidence from a 20-year-old case was handled by ungloved police officers or evidence custodians (prior to knowledge regarding the sensitivity of modern DNA testing), then the true perpetrator's DNA might be masked by contamination from the collecting officer. Thus, when a DNA test is performed, the police officer's or evidence custodian's DNA would be detected rather than the true perpetrator. In the absence of other evidence, the individual in prison might then be falsely declared 'innocent' because his DNA profile was not found on the original crime scene evidence. This scenario emphasizes the importance of considering DNA evidence as an investigative tool within the context of a case rather than the sole absolute proof of guilt or innocence.
To reduce contamination problems during the laboratory examination of DNA samples, all pre-PCR and post-PCR amplification reactions should be kept physically separate (see Chapter 4). Laboratory contamination is probably impossible to avoid completely but can be proactively assessed with negative controls and staff elimination databases (Gill and Kirkham 2004). However, if the original DNA sample has been contaminated during the collection processes (see Chapter 3), then the DNA profile would be a mixture, and the results would need to be interpreted as will be described in the next section.
Mixtures arise when two or more individuals contribute to the sample being tested. Mixtures can be challenging to detect and interpret without extensive experience and careful training. As detection technologies have become more sensitive with PCR sensitivity coupled with fluorescent measurements, the ability to see minor components in the DNA profile of mixed samples has improved dramatically over what was available with RFLP methods only a few years ago. Likewise, the theoretical aspects of statistical calculations for mixture interpretation have been examined more thoroughly (Curran et al. 1999). Some statistical approaches to reporting a mixture result will be discussed in Chapter 22.
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