Bloodstains are an important piece of evidence in a forensic examination. The pattern of a stain and the quantity of blood present can be important clues to the nature of the accident or crime. Moreover, detailed analysis of the blood obtained from a stain can reveal genetic and other information that can help identify a victim or implicate the person responsible.
Analysis of bloodstains can also help reveal the nature of the injury and even the order that the wounds were received. The pattern of the bloodstain, which is also referred to as blood spatter , can be important in identifying the weapon used to inflict the injury, and help determine if the victim was moving or motionless when injured.
When initially dealing with a bloodstain, a forensic investigator will seek to obtain as much information as possible without disturbing the scene. This can involve recording the bloodstain by means of a sketch, video camera, or digital camera. As well, a chemical called luminol can be sprayed over the area of the bloodstain. Under ultraviolet illumination, the luminol that has bound to blood will fluoresce, which can reveal small quantities of blood that might otherwise escape detection.
When establishing the extent and pattern of a bloodstain, the use of sufficient illumination is important, to avoid shadows that might contribute to misleading shapes to blood drops, since the shape of a drop can indicate the direction and speed of impact of the drop with the surface.
The pattern of a bloodstain can tell a lot about the origin of the blood. For example, blood can drip or ooze out of a cut or bullet wound, spurt out at much higher speed from a severed artery, or be flung off a weapon as a blow is delivered. The resulting patterns will differ. Indeed, the pattern of cast-off blood from a weapon can even be used to determine if the assailant was right- or left-handed.
A drop of blood falling from 5–6 feet (1.5–2 m) above a floor will splash upon impact, while a drop falling several feet straight down will tend to be somewhat circular but with a wavy edge. A drop oozing out of a cut that is just a few inches above a floor likely will be circular.
In another example, the high impact of a gunshot wound can send blood away from a body at high speed. If the blood impacts a wall at an angle, the blood drop will assume more of a teardrop shape than if the blood impacts the wall straight on. Moreover, the orientation of several of the teardrop-shaped bloodstains will allow a forensic investigator to apply trigonometric functions to produce a three-dimensional recreation of the area that the blood came from.
A smeared bloodstain around a body can be evidence that the body was dragged to that position. Similarly, a trail of blood drops leading to a body can be evidence that the person was moving while bleeding, or was being carried by someone else.
A skilled forensic investigator is often able to trace these patterns back to their origin; literally, to the scene of the crime.
Bloodstains can also be a treasure trove of other information. If someone walked through a bloodstain on the floor, an impression of the sole of the shoe may have been left. This piece of evidence can help match someone to the scene of the crime or accident. The blood may also have carried bits of skin, hair, or clothing with it. All these materials can be recovered and analyzed to provide more information about the victim or the assailant.
Blood naturally carries the genetic information of the person from which it originated. Blood cells will contain deoxyribonucleic acid (DNA ). The DNA can be enzymatically digested into many differently sized fragments and the fragments can be analyzed to deduce the sequence of building blocks (bases) that comprise the DNA. Small stretches of DNA that are of particular interest can be obtained in large quantities using a procedure known as the polymerase chain reaction (PCR ). Essentially, PCR makes a copy of the target region, uses those copies to make more copies, and so on. The number of copies grows at a logarithmic rate to quickly generate millions of copies of the target region. The unique region can then be studied.
In this way, an individual's DNA sequence can be found. This sequence, like a fingerprint, can be almost unique to the individual. DNA sequence analysis, if properly done, can be a powerful piece of evidence.
Such was the case in the O. J. Simpson murder trial. The DNA pattern obtained from blood at the scene of the double murder matched the patterns obtained from blood found in Simpson's vehicle and from a blood sample obtained from Simpson himself. The odds that the blood was Simpson's and not someone else's were astronomically high. However, Simpson's lawyers were able to create doubt in the juror's minds concerning the way the blood samples were collected. The result, despite this overwhelming evidence, was an acquittal.
A bloodstain can also be tested to determine the blood type. There are four possible blood types: A, B, AB, and O. These various types can be distinguished from one another by virtue of the different proteins (antigens) on the surface of the blood cells. The different antigens can be recognized by specific antibodies.
Blood typing can be another powerful means of linking someone to a crime or accident scene, or exonerating them. For example, if a bloodstain at a crime scene contains Type A blood and a suspect has Type O blood, then the blood did not come from the suspect. However, because many people have the same blood type, blood typing alone cannot implicate someone.
Blood also can possess another antigen group called the Rh factor. Individuals who produce the Rh antigen have Rh positive blood. Those who do not produce Rh antigen have Rh negative blood.
Bloodstain analysis is now recognized as a vital facet of a forensic examination. This importance is exemplified by the Royal Canadian Mounted Police (RCMP), who maintain a Crime Scene Bloodstain Section in their Forensic Support Services. This section is one of the most advanced in the world, and is the only agency that specifically trains investigators to do bloodstain analyses at a crime scene.
see also Blood, presumptive test; Blood spatter; Blood volume test; DNA fingerprint; Indicator, acid-base; Simpson (O. J.) murder trial.