TRACE DNA ANALYSIS

Trace DNA has become a large part of the average forensic laboratories' workload. Remarkably low DNA amounts (<100pg) have been successfully analysed to obtain profiles from a wide range of sample types. Touched objects constitute the most common source of trace DNA, but any type of biological material present in low amounts may be considered as trace, including minute blood deposits, saliva residue on partially consumed food, or even epithelial cells from the interior surface of condoms. In addition to supplementing existing analysis techniques in serious crime cases, trace DNA can allow investigation of volume crime cases such as burglary or vehicle theft, where DNA evidence had not previously been considered usable. However, despite the widespread use of trace DNA, at present there are very few specific validated methods. This has ledto controversy in the use of trace DNA, and particularly the low copy number amplification technique. It has been established that the use of existing methodology (developed for high-copy number samples) leads to significant levels of artefacts with trace DNA, including allele drop-out and drop-in, stutter, and allelic/locus imbalance. To minimize these, there are numerous modifications that can be made to existing methods to increase the success of trace DNA analysis. These include reduced extraction volumes, increased cycle number, reduced PCR volume, and increased injection time for capillary electrophoresis. In addition, new research shows that the introduction of techniques such as whole genome amplification, molecular crowding, and post-PCR purification can significantly increase success rates with trace DNA. It is clear that each step of the analysis procedure (including collection, extraction, amplification and fragment detection) can, and should, be optimized with regards to trace DNA. However, the use of increasingly sensitive trace DNA analysis techniques must bring an increasing awareness of the potential for contamination, both within the laboratory and at crime scenes. In particular, this has implications for the analysis of trace DNA from cold cases, which were not collected or stored with highly sensitive DNA detection techniques in mind. Although it is certain that trace DNA will continue to be used within forensic biology, there may need to be modifications to a wide range of practices to ensure accuracy and reliability.