Blood is a vital liquid tissue of the body and consists of the plasma and the blood cells. Blood evidence can provide information regarding the crime, the victim, the perpetrator, as well as the events surrounding the crime. It is important to establish the presence or absence of blood, whether it is of human or animal origin and, if animal, the species thereof. Blood of human origins then needs to be examined for its DNA composition. Blood typing based on the A-B-O blood types is no longer of much service in crime scene investigation. DNA analysis is extremely accurate and has replaced whole blood typing.
At best whole blood typing “excludes” a perpetrator from a scene, rather than undeniably “identifying” the donor. This is done by means of a complex system of A-B-O antigen-antibody interactions and other biochemical character-istics that characterize human blood. Blood stains are useful in reconstructing the crime scene. The study of blood patterns is a visual exercise which requires practical experience and interpretation, in context of the crime scene, by a skilled investigator. Bloodstain convergence refers to the common point of origin from which a group of blood stains originated.
Droplets of blood striking a surface will leave a characteristic pattern indicating the direction in which they were travelling. Lines projected through their directional axes will indicate roughly their common point of origin (Turner, 1965 p 135). Several tests such as the Kastle-Meyer (color) and luminol (luminescent) tests exist to determine the existence of blood at a scene. If properly performed these tests leave little doubt as to the presence or absence of blood, visible or not to the naked eye. A negative test is absolute proof of the absence of detectable quantities of blood.
Any of the tests by color or luminescence can be performed readily in field conditions. References Kirk, P. L. (1953). Crime Investigation. Physical Evidence and the Police Laboratory (pp. 176-203). New York: Interscience Publishers. Ward, R. H. (1975). Introduction to Criminal Investigation (p. 63). California: Addison-Wesley Publishing Company. Eckert, W. G. (Ed. ). Introduction to Forensic Sciences (pp. 155-163). London: The C. V. Mosby Company. Turner, W. W. (Consulting Ed. ). Case Investigation Part I (p. 135). California: Aqueduct Books. A Division of The Lawyers Co-operative Publishing Company.