Saturday, September 25, 2010

Partial Profiles

Use of "partial profiles" is a newly emerging and fairly disturbing trend.  A partial profile is one in which not all of the loci targeted show up in the sample.  For example, if 13 loci were targeted, and only 9 could be reported, that would be termed, a partial profile.  Failure of all targeted loci to show up demonstrates a serious deficiency in the sample.  Normally, all human cells (except red blood cells and cells called "platelets") have all 13 loci.  Therefore, a partial profile represents the equivalent of less than a single human cell.  This presents some important problems: 
1.         A partial profile essentially proves that one is operating outside of well-characterized and recommended limits. 
2.         Contaminating DNA usually presents as a partial profile, although not always.  For this reason, the risk that the result is a contaminant is greater than for samples that present as full profiles.
3.         A partial profile is at risk of being incomplete and misleading.  The partial nature of it proves that DNA molecules have been missed.  There is no way of firmly determining what the complete profile would have been, except by seeking other samples that may present a full profile. 
Most forensic laboratories will try to obtain full profiles.  Unfortunately, in an important case, it may be tempting to use a partial profile, especially if that is all that one has.  However, such profiles should be viewed skeptically.  Over-interpretation of partial profiles can probably lead to serious mistakes.  Such mistakes could include false inclusions and false exclusions, alike.  It could be said that, compared to the first PCR-based tests introduced into the courts, use of partial profiles represents a decline in standards.  This is because those earlier tests, while less discriminating, had controls (known as "control dots") that helped prevent the use of partial profiles.  The earlier tests will be discussed below, primarily for historic reasons, but also because they do still appear on occasion.

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