Appendix A: The Role Of Serology
Forensic serology is best defined as the science involving the identification and characterization of blood, semen, and other body fluids, usually found in dried stain form on items of physical evidence. Because of its supportive nature to the prosecution, it is absolutely essential that the investigators and the prosecutor understand, at least in general terms, the capabilities,as well as the limitations, of forensic serology.
Under most conventional serology protocols, items of physical evidence in sexual assault cases are scientifically screened for the presence of human semen and blood. Semen is identified where sperm cells are microscopically identified and/or a semen-specific protein associated with human semen, known as p30 or prostatic antigen, is determined to be present in extracts of dried stains under examination.
Once the presence of semen is established, the stain extracts may be analyzed for the presence or absence of blood group substances. Eighty percent of the general population secrete chemicals (blood group substances) into their body fluids that are consistent with their red blood cell ABO type. In these people, known as secretors, analysis of their saliva, vaginal secretions,or semen will indicate whether they belong to blood group A, B, AB or O.
If human blood is identified on an item, attempts may be made to determine whether the blood is of A, B, AB or O. Then, depending upon the size of the dried stain, it may be analyzed using electrophoresis to determine as many genetic marker protein types as possible. In order for any of this to make sense, known blood and saliva samples from both the victim and suspect of the sexual assault must be submitted for analysis and comparison purposes.
The known blood samples may be grouped as to A-B-O blood type and assorted genetic enzyme markers. The serologist may also attempt to determine if the individual is a secretor from analysis of the liquid blood and/or dried saliva standard.
Once all of the scientific information is assembled, the serology examiner may be able to make expert conclusions from the findings. Forensic serology is a comparison science. If all of the information from the analysis of the questioned samples is identical to that obtained from the known blood and saliva samples from the suspect, then the serologist can conclude that the suspect was a possible source of the deposited semen or blood stain. If, however, one element of the profile differs, then the serologist may be able to absolutely exclude the suspect as a source of that body fluid. It is further emphasized that without known standards from the victim, suspect(s) and any other involved parties for comparison, the effective serological analysis of items of evidence from the assault may be rendered meaningless.
Using this technology, the best that a serologist may be able to say is that the suspect is a possible depositor of the body fluid. This is because other potential suspects in the general population may share the same A-B-O blood type, secretor status, and enzyme profile, although the implementation of DNA testing in forensic samples has dramatically altered this thinking.
Appendix B: DNA Profiling
Deoxyribonucleic acid (DNA) is an organic substance found primarily in the chromosomes within the nuclei of cells. Using electrophoresis and radioactive probing techniques, a DNA profile can be developed from dried blood and semen stains.
DNA profiling is the FBI Laboratory’s primary method of choice for the serological analysis of physical evidence from violent personal crimes. This technology has revolutionized the processing of serological evidence and has superseded traditional serology techniques in its associative value.
Currently, the FBI Laboratory screens probative items of evidence in sexual assault matters, such as vaginal, oral, and anal swabs and panties from the victim, for the presence of semen. At the conclusion of this analysis, the remaining stain may be submitted for DNA profiling. If human semen is identified,the sample is submitted for DNA analysis.
DNA profiles in the questioned samples are compared side by side with the DNA profiles in the known blood samples from the victim and suspect. If the patterns on the suspect’s blood profile match those generated from the questioned samples, the serologist can testify that the semen or blood was deposited by the suspect or a member of a group of individuals who share this profile. As with conventional serology, if DNA profiles do not match, the serologist can absolutely exclude the suspect as the contributor of that DNA sample.
Many U.S. crime laboratories do not possess the capability to conduct DNA testing. Therefore, investigators and prosecutors should become familiar with the capabilities of their local laboratories in this regard. The current protocol used at the FBI Laboratory necessitates an 8- to 10-week time period from initial DNA extraction to final probing results in each case sample. Additional delays may also exist due to high caseload and the requirement for analysis in other forensic disciplines. DNA analysis may be more time-consuming and labor intensive than traditional serology techniques; however, the results may be far more significant or informative. The law enforcement community must be aware that DNA analysis of forensic samples is a lengthy process and trial continuances may be necessary.