DNA Evidence Basics: Possible Results from Testing
When the results obtained from the standard sample from a known individual are all consistent with or are all present in the results from the unknown crime scene sample, then the results are considered an inclusion or nonexclusion. The term "match" is also commonly used when the test results are consistent with the results from a known individual. That individual is included (cannot be excluded) as a possible source of the DNA found in the sample. Often, statistical frequencies regarding the rarity of the particular set of genetic information observed in the unknown evidence sample and for a known individual are provided for various population groups.
It is possible for a falsely accused individual to be included as a source of a sample, particularly if the test system used only tests at one or a few loci (e.g., the DQa). In this situation, additional testing at more loci should be performed with the remaining evidence and/or DNA.
In some cases where inclusions are reported, the results are not meaningful or are inconclusive for that particular case from a legal perspective. Situations where this might apply are when the results obtained are all consistent with the individual from whom the samples were collected (e.g., victim's results only on vaginal swabs taken from the victim, defendant's results only on a bloodstain on defendant's clothing).
When the results obtained from the standard sample from a known individual are not all present in the results from the unknown crime scene sample, the results are considered an exclusion, a nonmatch, or noninclusion. With limited exceptions, an exclusion of an individual at any one genetic region eliminates that individual as a source of the DNA found in the sample.
In some cases where an exclusion is reported, it may be necessary to do additional testing for that exclusion to be meaningful to the case or to provide evidence for exoneration. A situation where this might apply is when the defendant is excluded as a donor of the DNA in a sexual assault case, but no samples are available from the victim and/or consensual partners.
Results may be interpreted as inconclusive for several reasons. These include situations where no results or only partial results are obtained from the sample due to the limited amount of suitable human DNA or where results are obtained from an unknown crime scene sample but there are no samples from known individuals available for comparison. In the latter case, the results would be suitable for comparison once an appropriate sample for comparison is tested.