Blood residue and bloodstains
Blood residue and bloodstains (Forensic Science)
Blood consists principally of plasma and blood cells. Plasma is the yellowish fluid that carries suspended blood cells called erythrocytes and leukocytes. Erythrocytes, commonly known as red blood cells, get their color from the hemoglobin that carries oxygen from the lungs to the organs and periphery of the body. Mammalian red blood cells do not have nuclei and do not contain DNA (deoxyribonucleic acid), but they do have antigens on their outer membranes that can be used to type the blood. Leukocytes, commonly known as white blood cells, do contain nuclei and do contain DNA that is unique to an individual (with the exception of identical twins, who carry identical DNA); this DNA can be isolated and characterized to identify the source of the blood.
Because blood accounts for 8 percent of a healthy person’s weight, typically 5 liters (a little more than 5 quarts), and it circulates near the surface of the skin, almost all kinds of trauma to the body result in the loss of blood. At crime scenes, blood’s red color generally makes it readily apparent; in cases where attempts have been made to remove it, residues are difficult to eliminate completely. Blood residue has been identified on 100,000-year-old stone tools, and bloodstains left by Confederate soldiers wounded at the Battle of Gettysburg in 1863 have been recovered from between the floorboards of an attic where the soldiers had been hiding.
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Examining a Crime Scene (Forensic Science)
A dried, but relatively fresh, bloodstain is generally reddish-brown in color and glossy. The gloss eventually disappears under the action of sunlight, wet weather, or attempts to remove the stain, and the color turns gray. The color and gloss of blood may also be affected by the surface on which it is found.
At a crime scene, investigators’ search for bloodstains is facilitated by the use of flashlights, which are held so that the light falls at an angle to the surfaces being examined. Presumptive tests for blood are sometimes used when small quantities of fluids suspected to be blood are present, especially if it appears that an attempt has been made to clean the area; these tests are also used to differentiate blood from other stains, such as rust, ketchup, or chocolate syrup.
A crime scene search for blood extends beyond the immediate area of the crime, because bloody fingerprints may have been left in other areas, such as on doorknobs, drawers, or sinks. Towels, draperies, or other fabrics may have been used to wipe blood off hands. If a floor has been cleaned, blood may be found in the cracks or joints in the floor or under the edges of carpets or linoleum. Investigators must search clothing at the scene carefully; even clothes that have been cleaned can contain blood residue in seams or linings, or inside sleeves or pockets. Persons who were present at the scene during the crime may also have...
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Description and Recording of Bloodstains (Forensic Science)
Crime scene investigators record descriptions of all bloodstains found, including information on their forms, colors, sizes, and positions. Information on the physical appearance of bloodstains is best preserved through photography; photographs are usually taken at wide range, medium range, and close up. A scale is included in all close-up shots to show the sizes of the bloodstains or drops pictured.
A rough sketch of the crime scene is also often created to show the relationship of the bloodstains and other blood residue to other elements of the scene. At violent crime scenes, blood spatter evidence is often present, and its analysis can be invaluable in reconstructing how the crime occurred.
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Collection and Preservation (Forensic Science)
After crime scene bloodstain patterns and distribution have been well documented, the collection and preservation of blood residue and stains may proceed. Because these substances present the possibility of blood-borne disease, such as hepatitis or human immunodeficiency virus (HIV), investigators must be careful to protect themselves from infection. Also, they must take proper care to avoid contaminating the scene or cross-contaminating the samples collected. For collecting blood and other biological samples, investigators wear multiple layers of latex gloves, which they change frequently. Clean equipment is also essential to prevent contamination.
Blood is fragile, and to maintain its properties, investigators must ensure that blood evidence samples are properly preserved. If wet or damp bloodstains are stored in airtight containers, the blood will putrefy and be useless for forensic examination in a matter of days. In contrast, air-dried samples stored at room temperature or, better, under refrigeration will retain their usefulness for a much longer period. Ideally, biological samples should be stored in a frozen state, especially if they cannot be analyzed immediately.
Wet blood may be collected with a sterile disposable pipette or syringe and placed in a tube containing an anticoagulant to keep it from clotting any further. Alternatively, wet blood may be collected from pools of liquid blood with...
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Blood Typing (Forensic Science)
If presumptive tests indicate the presence of blood at a crime scene, further tests are performed to establish whether it is of human or animal origin. Reaction to appropriate antibody serum definitely establishes the species of origin. In addition, the blood group to which a human blood sample belongs can be established. Although more than twenty-nine human blood group systems are known, the ABO and Rh (or rhesus) groups are commonly used. With respect to the former, blood may be typed as A, B, AB, or O, characterized by the presence of antigen A, antigen B, both antigens, or neither antigen on the surface of red blood cells, respectively. With respect to the Rh group, a sample is either positive or negative for Rh antigen. A person’s blood type is inherited and hence unchangeable. Although ABO and Rh blood group analysis does not link a sample to a particular person, it can enable investigators to include or exclude a person of interest as a suspect or victim. Because blood group antigens deteriorate with age or improper storage, samples that have not been collected and stored with care often cannot be typed.
Blood also contains DNA, however, which is less subject to deterioration. Given that DNA testing of bloodstains and other blood residues can provide positive identification of the source of the blood, law-enforcement agencies around the world rely on DNA analysis of any blood and other biological samples recovered from...
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Further Reading (Forensic Science)
Fisher, Barry A. J. Techniques of Crime Scene Investigation. 7th ed. Boca Raton, Fla.: CRC Press, 2004. Standard text in the field includes a chapter on blood and other biological evidence.
Geberth, Vernon J. Practical Homicide Investigation: Tactics, Procedures, and Forensic Techniques. 4th ed. Boca Raton, Fla.: CRC Press, 2006. Text used in many U.S. police academies provides full coverage of all aspects of homicide investigations, including the collection of blood evidence.
Lee, Henry C., Timothy Palmbach, and Marilyn T. Miller. Henry Lee’s Crime Scene Handbook. San Diego, Calif.: Academic Press, 2001. Practical guide to crime scene procedures includes a section on the collection, preservation, and analysis of blood.
Tilstone, William J., Kathleen A. Savage, and Leigh A. Clark. Forensic Science: An Encyclopedia of History, Methods, and Techniques. Santa Barbara, Calif.: ABC-CLIO, 2006. Comprehensive work covers the role of blood analysis in forensic investigations, including blood groups, bloodstain identification, blood spatter analysis, and presumptive tests for blood.
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