Forensics of Blood Blood evidence is the most common, most recognized and possibly most important evidence in the world of criminal justice. Throughout the history of violent and fatal crimes, bloodstain evidence has recently begun to emerge as a recognized forensic skill. “Blood is one of the most significant and frequently encountered types of physical evidence associated with forensic investigation of death and violent crime” (Eckert & James, 11). When a violent crime has been committed, blood is commonly left behind at the scene of the crime. This blood evidence allows police investigators to piece together the events of the crime. The texture, size, shape, and distribution of the blood can be used to determine when the crime occurred, what weapon was used, how the victim was standing, how many times the victim was hit, and more. Even if the perpetrator attempted to cover up the crime by cleaning the blood, evidence of blood still remains. Forensic investigators have different tests that can see where blood has been, whether it is blood, saliva, or semen, and also can test whether the blood is even human or not. Going even further, more tests can be done to compare the blood of different individuals to determine whether or not they were the perpetrator.
Within the blood, there is a liquid portion called plasma making up about 55% of the total volume of blood. The plasma contains mostly water with some nutrients, minerals, and oxygen. In the plasma, there are red and white blood cells. Red blood cells contain hemoglobin which is a complex molecule that carries oxygen, removes carbon dioxide, and gives blood its red color. White blood cells; however, are crucial for the body’s immune system as they fight off infections by attacking harmful bacteria and viruses.
Analysis of bloodstain evidence is very important in an investigation of a crime, thus many crime scenes are reconstructed with the evidence found so as to gain a better understanding to what occurred during the attack. “Many sources of variability arise with the production of bloodstain patterns, and their interpretation is not nearly as straightforward as the process implies. Interpreting and integrating bloodstain patterns into a reconstruction requires, at minimum:
An appropriate scientific education
Knowledge of the terminology employed (e.g., angle of impact, arterial spurting, back spatter, castoff pattern);
An understanding of the limitations of the measurement tools used to make bloodstain pattern measurements (e.g, calculators, software, lasers, protractors);
An understanding of applied mathematics and the use of significant figures;
An understanding of the physics of fluid transfer;
An understanding of pathology wounds; and
An understanding of the general patterns blood makes after leaving the human body” (National Research Council, 177).
“Relative to the reconstruction of a crime scene, bloodstain interpretation may provide information to the investigator in many areas: 1. Origin(s) of the bloodstains. 2. Distances between impact areas of blood spatter and origin at time of bloodshed. 3. Type and direction of impact that produced bloodstain or spatter. 4. Object(s) that produced particular bloodstain patterns. 5. Number of blows, shots, etc. that occurred. 6. Position of victim, assailant, or objects at the scene during bloodshed. 7. Movement and direction of victim, assailant, or objects at scene after bloodshed. 8. Support or contradiction of statements given by suspect or witnesses. 9. Additional criteria for estimation of postmortem interval. 10. Correlation with other laboratory and pathology finding relevant to the investigation” (Eckert & James, 13).
After a crime, the perpetrator may attempt to clean up the blood from the scene. Although the blood may not be visible anymore, tests can be done in order to reveal the remaining traces of blood. Luminol is a chemical that displays chemiluminescence when combined with a suitable oxidizing agent. Luminol is a presumptive test that can detect traces of blood left at the scene of the crime, even after cleaning the area. After spraying luminol, in a darkened room, the luminol will react with the iron in the hemoglobin of the blood which catalyzes the luminescence. This reaction causes the emission of a blue glow lasting for approximately 30 seconds. Although luminol is useful in detecting small traces of blood, there are some disadvantages to using it. One disadvantage is the sensitivity of the solution and the blood itself. “75 years ago, luminol was introduced to the forensic field as a blood search technique. Since its introduction, much investigation has been done to enhance the solution in order to get a longer and more intense/brighter luminescence reaction with the blood. Besides the importance of the pH-level of the solution, most of these investigations came to the conclusion that adding more chemicals and/or solution enhances the brightness and duration of the reaction. However, this process also potentially damages the DNA and bloodstain pattern itself. Up till now, hardly any investigation has been done into the enhancement of the catalyst in this reaction process: the blood (haemoglobin) itself” (Eversdijk, 12). The chemiluminescence of luminol can occasionally be activated by compounds containing copper, bleaching chemicals, animal blood, fecal matter, semen, and even saliva. To determine if a substance is blood, forensic investigators can perform other tests. The Kastle-Meyer test is another presumptive blood test. A chemical indicator called phenolphthalein is used to detect the presence of hemoglobin. An alleged sample of blood is collected on a swab and a drop of the phenolphthalein is added to the sample along with a drop of hydrogen peroxide. If the swab immediately reacts and changes color to a bright pink, the sample is presumed to be a positive sample containing blood. Similar to the luminol test, the Kastle-Meyer test also has limitations. Chemicals such as copper and nickel salts will turn the swab pink before the addition of hydrogen peroxide. This is why it is important to wait before adding the hydrogen peroxide a few seconds after the addition of the phenolphthalein. Another limitation is that the Kastle-Meyer test will react with any hemoglobin based blood sample, which could be human or animal. The luminol test and Kastle-Meyer tests are both presumptive tests, meaning that a positive result only tells that it is possible there is blood in the sample. In order to be 100% positive that the sample contains blood, confirmatory tests must be performed. “The identification of blood can be made more specific if microcrystalline tests are performed on the material…Crystal tests are far less sensitive than color tests for blood identification and are more susceptible to interference from contaminants that may be present in the stain” (Richard Saferstein, 249). For example, the Takayama crystal test is performed by “the application of a specific solution developed by Takayama” to a small amount of blood. After the blood has been treated with the solution, hemochromogen crystals begin to form (“Serology-Blood and Other Bodily Fluids”). The iron in hemoglobin reacts with pyridine to produce red feathery crystals. Another test is the Teichman test, which uses a solution of potassium bromide, potassium chloride, and potassium iodide in acetic acid. This solution is heated to react with hemoglobin. The hemoglobin is first converted to hemin, and then the halides react with the hemin to produce yellow-brown rhomboid shaped crystals. After concluding that a sample truly contains blood, the question at hand is whether or not the blood is of human origin. “One of the most widely used tests [to determine if the sample is human blood] is called the precipitin test, in which the presence of human blood is revealed by making it clot” (“The Forensics of Blood”). Precipitin is an antibody that reacts with its corresponding antigen to form a precipitate. The precipitin test is based on the fact that when animals are injected with human blood, antibodies form that react with the invading human blood in order to neutralize its existence. An investigator can recover these antibodies by bleeding the animal and isolating the blood serum. The serum, containing antibodies that react specifically with human antigens, is called human antiserum. There are a number of techniques that have been devised for performing precipitin tests on blood samples. “The classic method is to layer an extract of the blood sample on top of the human antiserum in a capillary tube. Human blood…reacts specifically with antibodies present in the antiserum, as indicated by the formation of a cloudy ring or band at the interface of the two liquids” (Richard Saferstein, 250). Gel diffusion is another way to perform the precipitin test. This process takes advantage of the fact that antibodies and antigens diffuse towards one another on an agar plate. The blood sample and the human antiserum are placed in holes at opposite ends of the gel. If the blood sample in question is of human origin, a line of precipitation will form where the antigens and antibodies meet. In the electrophoretic method, an electric potential is applied to the gel medium. The reaction between the antigens and antibodies is represented by a precipitation line between the hole containing the blood sample and the hole containing the human antiserum. The precipitin test requires only a small amount of blood and human blood dried for 15 years may still give a positive result with the precipitin test. Once a blood sample has been determined to be of human origin, “an effort must be made to associate or disassociate the stain with a particular individual” (Richard Saferstein, 250). This is done by blood typing and DNA analysis. An investigator can conclude whether blood collected from the crime scene belongs to the victim, the criminal, or another person who may have been involved by identifying blood type. The four blood types are A, B, AB, and O and they are defined based on which proteins are present on the surface of an individual’s red blood cells. There are two main antigens present on the surface of red blood cells called A and B antigens. “Another important blood antigen has been designated as the Rh factor, or D antigen. People with the D antigen are said to be Rh positive; those without this antigen are Rh negative” (Richard Saferstein, 243). “Some people have only A antigens on their red blood cells, some have only B antigens, some others have both A and B antigens, and others have none, making these people’s blood of type A, B, AB, and O, respectively” (“The Forensics of Blood”). Blood contains antibodies that attach to these antigens, however, antibodies only attach to foreign antigens. For example, type A blood contains A antigens and anti-B antibodies that bind only to B antigens. Antibodies are generally bivalent, meaning they have two reactive sites where the antibody can simultaneously be attached to antigens located on two different red blood cells. This creates cross-linked cells and is called agglutination. Investigators can determine blood type by injecting either anti-A or anti-B antibodies and by observing whether the antibodies bind to the antigens. Blood type can also be determined by detecting antibodies rather than antigens. An investigator can use A and B cells to test for the presence of anti-A or anti-B antibodies. If A cells are added to a blood specimen, the cells bind with one another only in the presence of anti-A antibodies, therefore, the cells have B antigens on the surface of the red blood cells and the blood is determined to be of type B. If the blood type of a suspected criminal is different than the blood detected at the crime scene, then the individual is unlikely to have committed the crime. If the blood type of the suspected criminal does match that of the blood found at the crime scene, then it is possible that this individual committed the crime; however, it is not definitive, it is only a possibility. DNA analysis must be done in order to conclude that the suspected criminal did, in fact, commit the crime. Patterns of the blood stains can provide investigators with clues to reconstructing the crime. The size and shape of the blood can give information about how the individual was placed. “While blood has many physical properties that are similar to the properties of water, blood responds differently to the same external forces because blood has a different viscosity, adhesion, capillary action, and density. These factors, after examined in a laboratory, can be translated to crime scenes” (Bloodstain Pattern Analysis). Blood is a colloid, which is a fluid substance where very small particles of another substance are dispersed and are commonly sticky substances. In addition to being a colloid, blood is also viscous. Viscous substances tend to fall very slowly. Since blood is viscous and a colloid, it readily forms recognizable patterns on the surface on which it falls. An investigator can determine the direction of travel of blood striking an object by studying the blood stain’s shape. As the stain appears more elliptical in shape, the direction of impact becomes more apparent since the pointed, tail end of the blood stain indicates the direction of travel. If the blood is symmetrical, the blood fell perpendicular to the surface. To measure a blood stain dropped onto a flat surface, the degree of circular distortion must be measured. A drop of blood falling at a 90 degree angle will be circular with no tail, whereas a drop falling at a lower angle of impact, less than 90 degrees, will appear more elongated in shape. The more angled a droplet falls, the more oblong and oval-shaped the droplet will be. “The roughness and porosity of the surface on which the droplet has fallen also plays a key role--blood dropped onto concrete, for example, will tend to have a more jagged shape than blood dropped onto a softer or less porous surface” (“The Forensics of Blood”). Spatter patterns can assist investigators along in their analysis of the crime scene. Impact spatter is a blood stain pattern produced when an object makes forceful contact with a source of blood, projecting drops of blood outward from the source. This type of blood spatter is one of the most common types of blood stain patterns found at crime scenes. If the spatter is projected away from the source in the same direction as the force causing the spatter, it is called forward spatter. If the spatter is projected backwards toward the source of the force causing the spatter, it is called back spatter. Investigators have derived a system of classification for impact spatter from the velocity of a droplet of blood. As the force of impact on the source of blood increases, the velocity of the blood drops stemming from the source also increases. Also, generally, as force and velocity of impact increase, the diameter of the resulting blood drops decreases. An impact spatter created by a force traveling at 5 feet per second or less and producing drops with diameters greater than 3 millimeters is classified as low-velocity spatter and is usually caused by minimal force. An impact spatter created by a force traveling between 5 to 25 feet per second and producing drops with diameters between 1 and 3 millimeters is classified as medium-velocity spatter and is normally associated with blunt-force trauma. An impact spatter created by a force travelling at 100 feet per second or faster and producing drops with diameters less than 1 millimeter are classified as high-velocity spatter and is commonly produced by gunshot exit wounds or explosions. “Using droplet size to classify impact patterns by velocity is a useful tool for giving investigators insight into the general nature of a crime. However, the velocity at which blood strikes a surface by itself cannot illuminate the specific events that produced the spatter pattern” (Richard Saferstein, 303). Generally, investigators should use velocity categories cautiously. Impact spatter can offer investigators some help in determining the origin of the blood source and the position of the victim at the time of impact. The area on a two-dimensional plane where lines traced through the long axis of several individual blood stains meet approximates the two-dimensional place from which the blood stains were projected. This area is known as the area of convergence. The approximate area of origin will be on a line straight out from this area. “An object hitting a source of blood numerous times will never produce exactly the same pattern each time. One may therefore determine the number of impacts by drawing the area of convergence for groups of stains from separate impacts” (Richard Saferstein, 304). The area of origin is the location in three-dimensional space that blood the blood produced a blood stain originated from. The location of the area of convergence and the angle of impact for each blood stain is used to approximate the area of origin. This will show the position the positioning of the victim or suspect in space when the force causing the impact occurred. As the distance from the surface increases, the distribution and distance between drops increases. The string method is a common method for approximating the area of origin with an error of 2 feet. This method is done by following these steps: 1. Find the area of convergence. 2. Place a pole at the area of convergence to act as the axis 3. Attach a string next to each droplet. Use a protractor to determine the angle of impact and attach the string next to the droplet to the pole in the same angle of the impact. 4. View the area of origin of the drops where the strings appear to meet. Secure these strings as the area of origin.
Other blood stain spatter patterns can possibly be caused by gunshot spatter, cast-off spatter, arterial spatter, expirated blood patterns, void patterns. Conversely to spatter patterns, “circumstances of the crime often create other types of stains that can be useful to investigators” (Richard Saferstein, 309). Transfer patterns are blood stain patterns created when a surface carrying wet blood comes in contact with a second surface. Examples of this type of pattern in blood stains include fingerprints, handprints, footprints, footwear prints, tool prints, and fabric prints. The imprints seen in a transfer pattern can help investigators narrow down the list of possible suspects or weapons by comparing the transfer pattern imprint with evidence collected. Flow patterns are made by drops of large amounts of blood flowing by the pull of gravity. These patterns may occur from single drops or large quantities of blood coming from an actively bleeding wound or blood deposited from an arterial spurt. The direction of the flow pattern can show movements of objects or bodies. Pools of blood occur when blood collects in a level, undisturbed place. The edges of the stain will dry to the surface it rests on and will skeletonize. This can be important for classifying the source of the original stain. Drip trail patterns are bloodstains that are formed by the dripping of blood off a moving surface or individual in a recognizable pathway separate from other patterns. This can be useful by allowing investigators to recreate the events of the crime. Blood analysis is a gruesome, detail-oriented process that requires much attention and precision, but it plays a crucial role in forensics. Blood can be used to determine so many different things about a crime. Even though many of the tests are tedious, analysis of blood can either incriminate an individual or it can disassociate an individual from the crime. This is why it is so important to be cautious during testing of blood. Any slight slip up in the lab can cause someone can be wrongfully incriminated. Another wonder of blood analysis is how much information it can provide about the events of a crime. An investigator can almost recreate the entirety of a crime simply by reading the story laid out by the blood stains.

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