Low Copy Number or Low Template DNA analysis
The Forensic Institute has been one of the main campaigners in bringing the limitations of the use of the Low Copy Number (LCN) DNA technique, and its failure to be internationally accepted, to the attention of UK courts.
The use of DNA profiling has revolutionised the use of science in legal cases. DNA profiles produced from manufactured kits such as SGMPlus and Identifilier are routinely used as evidence in criminal cases. The kits are designed and validated by the manufacturer to operate within a specific range of amounts of DNA, typically 0.5 – 2.5ng (a nanogram is a 1,000th of a millionth of a gram). The kits work by copying (amplifying) the DNA molecules contained in a sample a number of times to produce enough to be detected in the analyser. The chemistry used by the kits is capable of amplifying just one molecule of DNA. By varying the conditions under which the kit is used some claim to be able to produce profiles from much lower amounts of starting (template) DNA. The general term for these techniques is variously referred to as Low Copy Number (LCN) or Low Template DNA (LTDNA) analysis, although some restrict the term LCN to the process used by the FSS Ltd which uses 34 cycles of amplification instead of the routine 28-cycle process recommended by the manufacturer. However, with very low numbers of template DNA molecules the process may fail to amplify the template. This leads to a number of problems in the interpretation of the resulting profiles. These are caused mostly by sampling, or stochastic, errors caused by the failure of the chemistry to work effectively with such low numbers leading to poor reproducibility of the results.
In our view, the reliability of the LCN method below the stochastic threshold has not been demonstrated with sufficient numbers of samples and with samples which represent those likely to be discovered in crimestains. The limited extant data must be made available to enable the scientific community to conduct a meaningful assessment of the inferences that can be made from it.
By way of analogy, an aeroplane, designed on the principles of flight, will fly and perform satisfactorily within the parameters of its design. As the speed lowers, there is no change to the aeroplane’s performance according to the laws of aerodynamics, but as the speed lowers further the aircraft will first suffer a loss of control and then simply stop flying (this is termed the stall speed). A graph of the flying performance of the aircraft will be continuous, but will show a precipitous fall-off at the stall speed. The stochastic threshold is the DNA profiling equivalent of this sudden change in performance. Profiling above the stochastic threshold produces consistent and reliable results. Below the stochastic threshold, reliability fails. It is disingenuous to equate the performance of ANY profiling technique above the stochastic threshold, to the performance below; that now appears to have been recognised scientifically and legally.
