READ: Components of Blood

Immunoassay Techniques

When a stain is encountered at a crime scene that appears to be blood, there are several questions that must be answered:

1. Is it blood?

2. If it is blood, is it human or some other species?

3. If it is human, can it be linked to a particular individual?

Is it Blood?

 A presumptive, or screening, test for blood often used in crime scenes is the spraying of suspected areas of blood staining with Luminol. If blood was present at some point, the luminol will react with the blood stain and cause the area that was covered with blood to fluoresce. Luminol is simply a chemical reaction in which the Luminol solution reacts with iron from the hemoglobin of blood to cause a faint blue fluorescence. Even tiny amounts of iron can cause Luminol to fluoresce, so it is quite reliable in detecting trace amounts of blood. Because Luminol is extremely sensitive, it can detect even diluted samples of blood that were cleaned up long before. If luminol is sprayed in an area suspected of being a cleaned up crime scene, it will still detect blood that has been diluted up to 300,000 times. If the stain is blood, luminol will not interfere with any subsequent DNA analysis that may be done though it may destroy other components for testing within the blood.

 Another presumptive test is called the Kastle-Meyer Color Test. The Kastle-Meyer Color Test uses a solution of phenolphthalein and hydrogen peroxide on a piece of filter paper to detect blood. When blood of any quantity is present and exposed to the testing paper or solution, it will turn pink.

If it is blood, is it human or some other species?

When trying to determine if a blood stain is human or other animal in origin, it is important to know that various species produce different and distinct proteins and antibodies in the blood and body. Precipitin is an antibody that reacts with its corresponding antigen to form a precipitate if the blood is from a human. Therefore, the precipitin test is done to distinguish human blood from other animal blood. The basic principle of the Precipitin test is that a serum with anti-human antibodies is poured onto or mixed with a blood sample. If the blood sample clots, then the blood sample must be from a human. This type of serum is known as an "Anti-serum" and is created by injecting human blood into an animal, such as a rabbit, to allow the animal's blood to develop antibodies against the human blood. The antibodies are then extracted from the animal's blood serum and placed into the testing serum. This procedure can also be done to produce specific testing serums for all species of animals in addition to the human testing serum. If the blood is identified as human, DNA evidence may be able to link the blood to a specific individual. Blood evidence can also be tested for the presence of drugs or alcohol to yield more information about the person from whom the blood originated.

Georgia Virtual, Forensic Serology, CC BY-NC-SA 3.0