Click each question to show or hide the answers.
Biological material that was recently deposited at a crime scene will typically yield a full profile that contains results for all STR markers tested; this can be used for comparing to reference samples. However, some samples can prove difficult to analyze such as samples from a crime scene that contain DNA from more than one individual. This is especially true for low-level DNA samples retrieved from items such as firearms.
Typically there are three possible laboratory outcomes:
- If the DNA profiles from the evidentiary and known samples are consistent at each locus, laboratory analysts can interpret this finding as a “match,” “inclusion,” or “failure to exclude.”
- If the two profiles are not consistent at each locus, the finding can be interpreted as a “nonmatch” or “exclusion.”
- If there are insufficient data to support a conclusion, the finding is often referred to as “inconclusive.”
A DNA laboratory report typically includes the following information:
- Administrative information on the case, such as agency, file number, evidence item numbers, victim name and suspect name
- Date of the report and the name and signature of the reporting analyst
- Listing of all items of evidence examined, and type of methodology or technique used
- Listing of loci tested or amplification kit used
- Results of the examination and/or conclusions
- Interpretation of the resultant data
- Statements regarding the disposition of evidence (e.g., “after analysis, the evidence is stored in the laboratory”).
Reports will contain an interpretive statement which addresses whether DNA profiles from evidence samples could be associated with or excluded from:
- A known individual (suspect, victim, third party)
- Other evidence samples (scene samples, sexual assault evidence)
- Database samples (offenders, forensic unknowns, or missing persons)
If one or more of the known samples is consistent with any of the evidence samples (a match or inclusion), a statistic will be provided indicating the rarity of the evidentiary profile. A statistic is also provided if all loci of a partial profile match the known sample, but this is obviously not as strong as two full-profile matches.
Based on the statistic, the jury will determine the value of the evidence. A match at all 13 loci between an evidentiary sample and a known sample is strong evidence that the known individual deposited the evidentiary biological sample. If there is a match at only a few of the loci, the evidence is considerably weaker.
No-suspect Cases: Being able to collect and analyze a full DNA profile is powerful evidence, but it’s not always going to lead investigators to the perpetrator. There must be a matching profile available to compare it to—either in a database or from a known sample. There is no master database that contains everyone’s DNA information.
Partial Profiles: In cases where samples have very low quantities of DNA, are exposed to extreme environmental conditions or are not properly preserved, it may be difficult to obtain a full DNA profile and the test may only yield a partial profile. However, partial profiles may still be helpful in determining if an individual could be included or excluded in the investigation.
To ensure the most accurate analysis of evidence, the management of forensic laboratories puts in place policies and procedures that govern facilities and equipment, methods and procedures, and analyst qualifications and training. Depending on the state in which it operates, a crime laboratory may be required to achieve accreditation to verify that it meets quality standards. There are two internationally recognized accrediting programs focused on forensic laboratories: The American Society of Crime Laboratory Directors Laboratory Accreditation Board and ANSI-ASQ National Accreditation Board / FQS.
In addition to these quality guidelines, laboratories submitting data to the National DNA Index System (NDIS) must adhere to the FBI Quality Assurance Standards (QAS). These standards stipulate that all DNA casework be reviewed by a separate analyst, called a technical reviewer. This is then followed by an administrative review of the final report. Laboratories must undergo an external audit every two years to ensure their processes meet the FBI’s QAS.
The Scientific Working Group on DNA Analysis Methods provides input to the FBI on the QAS and publishes other guidance, such as interpretation guidelines, to assist laboratories.
Are there any misconceptions or anything else about DNA evidence that might be important to the non-scientist?
Due to the popularity of television crime dramas, several misconceptions may exist regarding the use of DNA evidence. For instance, when a fictional investigator on TV finds a matching DNA profile in a database, she will often be presented with a picture of the person’s driver’s license. In real life, CODIS does not contain or provide any personal information. To continue the investigation, she must coordinate with the agency that analyzed and uploaded the sample to obtain these details.
In addition, DNA analysis takes time to be done properly and every test must be reviewed for accuracy prior to release. In a busy crime laboratory, laboratories often have a goal of providing an analysis report within 30 days. However, this turnaround time will vary widely depending on numerous factors including number of other cases waiting to be analyzed; the number of laboratory personnel available; the type and amount of biological material submitted; the laboratory’s equipment and other factors.
To compare a profile to those in a state databank will typically take several weeks to process. Once entered into the database, profiles are continuously searched against new profiles as they are entered to see if they match.
Additionally, while the technology exists to analyze samples with only trace amounts of DNA, these samples may often not meet the requirements to be entered into the national level of CODIS. Finally, while DNA evidence can prove extremely valuable to an investigation, in some cases, other, more traditional forms of evidence can be even more valuable to solving and litigating a case.