Testing of biological evidence

Understand how biological evidence is tested.[1]  Semen, blood, vaginal secretions, saliva, vaginal epithelial cells, and other biological evidence may be identified and genetically typed by a crime lab. The information derived from the analysis can often help determine whether sexual contact occurred, provide information regarding the circumstances of the incident, and be compared to reference samples collected from patients and suspects. A primary method used by crime labs for testing biological evidence is DNA (deoxyribonucleic acid) analysis.[2] The most common form of DNA analysis used in crime labs for identification is called polymerase chain reaction (PCR). PCR allows the analysis of evidence samples of limited quality and quantity by making millions of copies of very small amounts of DNA. Using an advanced form of PCR testing called “short tandem repeats” (STR), the laboratory is able to generate a DNA profile, which can be compared to DNA from a suspect or a crime scene.[3] 
 
Distinguish patients’ DNA from suspects’ DNA.  Blood, buccal (inner cheek) swabbings, or saliva should be collected from patients for DNA analysis to distinguish their DNA from that of suspects.[4] (Procedures for collecting these samples are provided later in this chapter.) If the case is reported, patients’ biological samples and DNA profiles should be used only for investigation of the sexual assault, and their DNA profiles should not be inputted into CODIS.[5] Neither biological samples nor DNA profiles should be provided to law enforcement or prosecution for another case in which patients may be suspects, inadvertently given to health insurance carriers, or used for research purposes without patients’ consent.[6] Criminal justice agency policies should be in place and followed for the secure storage of biological samples and appropriate disposal of these samples and DNA profiles.

[1] Section drawn from Understanding DNA Evidence: A Guide for Victim Service Providers by the National Commission on the Future of DNA Evidence; the West Virginia Protocol for Responding to Victims of Sexual Assault, 2002, pp. 31–32; and the California Medical Protocol for Examination of Sexual Assault and Child Sexual Abuse Victims, 2001, p. 40. 
[2] DNA determines each person’s individual characteristics. An individual’s DNA is unique except in identical twins. DNA in the cell nucleus is genetic material inherited from biological parents. (Drawn from Arkansas’ Sexual Assault: A Hospital/Community Protocol for Forensic and Medical Examination, 2001.) 
[3] There is a concern that if DNA evidence is found, prosecutors may not utilize other evidence, especially when labs have limited resources. But because persons known to victims commit the vast majority of sexual assaults, DNA findings must be used in conjunction with other forensic evidence recovered, particularly when issues of consent arise. Law enforcement investigators and prosecutors should receive training on maximizing the use of all forensic evidence collected. 
[4] L. Ledray, SANE Development and Operation Guide, 1998, p. 65. 
[5] In the fall of 2003, legislation was introduced to implement the President’s DNA Initiative provisions that would bar the inclusion of elimination samples in CODIS. These samples include those obtained from sexual assault victims, as well as individuals with whom they had recent consensual sex prior to the exam.
[6] An exception is that a forensic lab may input frequency information related to the DNA profiles in its statistical database. Victims’ identity remains anonymous.


Go to the Next Section: Exam and Evidence Collection Procedures > Exposure to infectious materials and evidence contamination

Download printer-friendly versions