Low copy number DNA testing
The decision by Portuguese police to make Kate and Gerry McCann formal suspects in the disappearance of Madeleine may throw attention onto the process of 'low copy number' DNA testing.
Low copy number (LCN) testing is a sensitive, time-consuming form of DNA profiling which can be used on very small quantities of DNA. It has been reported that it was this technique which was used by the Forensic Science Service in Birmingham to examine DNA samples from the car hired by the McCanns.
However the history of the Forensic Science Service and its use of low copy number testing is .
The Â鶹¹ÙÍøÊ×Ò³Èë¿Ú has been trying since February under freedom of information to obtain a copy of an FSS report into cases where criminals may have escaped prosecution because the service apparently failed to use LCN tests properly. We have also been seeking other general assessments held by the FSS or the Police of low copy number profiling.
However the Forensic Science Service has refused to give us the information. We are now appealing the case to the Information Commissioner.
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The problem with LCN is that the techniques used to amplify the DNA are very effective, but necessarily crude. ANY DNA found will be amplified, and it is therefore very difficult, if not impossible, to distinguish between the original trace and any contamination. In more conventional DNA assays, this is easier, as the ratio of the different strands can be assessed, and the different sources weighted according to the case, the sample etc. This is simply not possible in LCN.