Crime Scene Investigator Network Newsletter

Communication is the key to effectively processing blood evidence. Clear and open communication must exist between a crime scene's first responding officer, the case detective, the crime scene investigator, the forensic scientist analyzing the evidence, and the assistant district attorney handling the case. Of prime importance is the communication between the crime scene investigator and the forensic scientist. A crime scene investigator should know the crime lab's capabilities, the methods of blood collection and preservation preferred by the crime lab, the investigative information relevant to the forensic scientist, and the type of reference samples required by the crime lab. This information may change periodically as technology changes, lab policies change, lab personnel change, or lab administrations change. The preferences of forensic scientists also vary from lab to lab. A good method of blood evidence collection for one forensic scientist may not be a good method for another forensic scientist. The crime scene investigator should meet regularly with his or her crime lab's forensic scientists to determine the most suitable manner for collecting and preserving blood evidence. This ensures that the evidence is collected efficiently and effectively.

The technological state of blood evidence analysis has rapidly advanced in the last 20 years. ln the early seventies, most crime labs relied upon the ABO blood grouping system to characterize bloodstains. This meant that the blood could have come from 4 to 49% of the population. In the 1990's, most crime labs are relying on DNA analysis to characterize bloodstains. A blood source can now be statistically narrowed down to one person out of several million or even several billion. A crime scene investigator should know which method or methods of bloodstain analysis are available from his or her crime lab, the FBI lab, and private labs.

Currently, bloodstain analysis falls into three broad categories. A crime lab may use one, two, or all three methods when analyzing blood. These three categories are:

Conventional serological analysis
Analysis of the proteins, enzymes, and antigens present in the blood. These substances are more susceptible to degradation than DNA and this type of testing usually requires a "large" sample (quarter size) in good condition for optimal results. This type of testing is rarely statistically individualizing.

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