Since blood evidence associated with a crime can provide information that may solve the case, it is essential to correctly document, collect, and preserve this type of evidence. Improperly handled blood evidence can weaken or destroy a potential source of facts in a case. Properly collected and preserved blood evidence can establish a strong link between an individual and a criminal act. Blood evidence or the lack of blood evidence can also be used to bolster or contradict a witness statement or any statements that the suspect may make.
Blood evidence can also point the investigator in the direction he or she needs to go to solve the case. If blood evidence is documented, collected, and stored suitably, it can be presented to a judge or jury several years from the time of the criminal act. Perhaps the most powerful application of blood evidence is the ability to absolutely eliminate a person as a potential suspect in a crime. 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. (1) 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.