hree types of results can occur in DNA testing: inclusion, exclusion, and inconclusive results. It is important that victim service providers, investigators, and prosecutors understand the meaning of these terms and be able to explain their implications. While conclusive results are very reliable, DNA findings can sometimes yield results that are difficult to interpret.
When the DNA profile of a known individual (a victim or suspect) matches the DNA profile from the crime scene evidence, the individual is "included" as a potential source of that evidence. However, the strength of this inclusion depends, in part, on the number of DNA locations examined (up to 13 locations can be examined) and the statistic reflecting how often the particular profile would be found in the general population. A DNA profile shown to occur rarely in the population (for example, 1 time in 5 million people) would more strongly suggest that the individual is the source of the biological evidence than would a more common DNA profile (for example, 1 time in 5,000 people). Increasing the number of DNA locations tested typically results in more powerful statistics. For this reason, several DNA locations are tested whenever possible.
n some cases, a DNA inclusion may provide information that is of limited value to the investigative process. For example, results from samples taken from the victim may be consistent with the DNA of the victim, such as vaginal evidence in sexual assault cases. In addition, if the suspect wore a condom during the assault, was aspermatic due to a vasectomy, or did not ejaculate after the assault, additional DNA profiles may not be obtained from the evidence. The results do not mean the suspect was not present and did not commit the crime-only that the substance tested did not come from the suspect. Additionally, inclusion does not necessarily mean a suspect is guilty.
When the DNA profile from an individual (a victim or suspect) does not match the DNA profile generated from the crime scene evidence, the referenced individual is "excluded" as the donor of the evidence. In some cases, it may be necessary to perform additional testing to establish the source of the DNA profile in the evidence. For example, a blood sample may be requested from the husband of a sexual assault victim to determine whether the DNA profile obtained from the vaginal swab is the result of a prior consensual act and not the assault. Exclusion does not necessarily mean a suspect is innocent.
Inconclusive results indicate that DNA testing did not produce information that would allow an individual to be either included or excluded as the source of the biological evidence. Inconclusive results can occur for many reasons. For example, even with sensitive PCR testing, the quality or quantity of DNA obtained from the biological evidence may be insufficient to produce definitive DNA typing results. Inconclusive results also can occur if the evidentiary sample contains a mixture of DNA from several individuals (for example, a sample taken from a victim of a gang rape). Even if the suspect's DNA profile is found in the biological evidence, the presence of DNA from other sources may prohibit the establishment of an inclusive or exclusive result. If there is more than one perpetrator or if in a sexual assault case the victim recently had consensual intercourse in which semen also may have been deposited in the victim's vaginal region, the results could contain profiles from more than one person. When this happens, it is often not possible to determine which specific types came from which donor. The suspect cannot be excluded as a possible donor of the DNA found in the evidence sample, but a more conclusive result may not be possible. These cases must be reported as inconclusive. As with all DNA results, inconclusive findings should be interpreted in the context of the other evidence in a case.