Bloodstain Pattern Analysis

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Bloodstain pattern analysis (BPA) is one of several specialties in the field of forensic science. The use of bloodstains as evidence is not new; however, the application of modern science has brought it to a higher level. New technologies, especially advances in DNA analysis, are available for detectives and criminologists to use in solving crimes and apprehending offenders.
The science of bloodstain pattern analysis applies scientific knowledge from other fields to solve practical problems. Bloodstain pattern analysis draws on the scientific disciplines of biology,chemistry, mathematics and physics. If an analyst follows a scientific process, this applied science can produce strong, solid evidence, making it an effective tool for investigators.

Results of BPA

Not every result of BPA will qualify as incontrovertible evidence, but the following are some things a bloodstain pattern analyst may be able to determine conclusively and state as fact:
  • Movement and direction of persons or objects while they were shedding blood.
  • Position of persons or objects during bloodshed.
  • Movement of persons or objects after bloodshed.
  • The mechanism or object used to create a specific pattern.
  • The direction a stain was traveling when it was deposited.
  • The area of origin of an impact pattern.
  • The minimum number of impacts during an incident.
  • The sequence of events.
A basic understanding of blood spatter analysis allows first responding officers and investigators alike to assist in correctly collecting and preserving bloodstain data at the scene. If they know what they have at the scene, then based on their department policy they should know what they need to do next. Bloodstain pattern analysis requires sufficient education and training to be an effective investigative technique, which not all law enforcement officers attending a crime scene will necessarily have.
Bloodstain analysts receive specialized training. The foundation course in bloodstain pattern analysis is the Basic Bloodstain Pattern Analysis Course. This is taught at many government and private institutions. The course criteria were developed by the International Association of Bloodstain Pattern Analysts (IABPA) with the following stated purpose:
A course of instruction designed for investigators, crime scene technicians, forensic technicians, and others involved in criminal and medical-legal investigations and crime scene analysis. The course is intended to develop a fundamental knowledge of the discipline of bloodstain pattern analysis. The course should illustrate to the student basic principals of bloodstain pattern analysis and the practical application of the discipline to actual casework. The course syllabus is not intended to create an "instant" expert.
Beyond this basic course are conferences, seminars, and courses such as the Maths and Physics for BPA and The Advance Bloodstain Pattern Analysis Course, both of which are provided by the Ontario Police College (OPC) and the Royal Canadian Mounted Police (RCMP). These two institutions also have Bloodstain Analyst Understudy programs. The International Association for Identification (IAI) provides its own certification in bloodstain pattern analysis.
In addition to formal study of the subject, practical experience and experimentation is paramount in the development of a skilled bloodstain pattern analyst.


Bloodstain pattern categories

There are several different thoughts on how to classify and define bloodstain patterns. The following is one accepted way of categorizing them based on the mechanism that created the stain. The three stain groups are: Passive, Projected, and Transfer/Contact. The definitions used below are from the suggested IABPA terminology list:


Passive bloodstains

Passive bloodstains are those stains created by the force of gravity.
  • Passive Drops - Bloodstain drop(s) created or formed by the force of gravity acting alone.
  • Drip Pattern - A bloodstain pattern which results from blood dripping into blood.
  • Flow Pattern - A change in the shape and direction of a bloodstain due to the influence of gravity or movement of the object.
  • Pool Pattern - A bloodstain pattern formed when a source of blood is stationary for a period of time.


Projected bloodstains

A projected stain occurs when some form of energy has been transferred to a blood source.
  • Low Velocity Impact Spatter (LVIS) - A bloodstain pattern that is caused by a low velocity impact\force to a blood source.
  • Medium Velocity Impact Spatter (MVIS) - A bloodstain pattern caused by a medium velocity impact\force to a blood source. A beating typically causes this type of spatter.
  • High Velocity Impact Spatter (HVIS) - A bloodstain pattern caused by a high velocity impact/force to a blood source such as that produced by gunshot or high-speed machinery.
  • Cast-Off Pattern - A bloodstain pattern created when blood is released or thrown from a blood-bearing object in motion.
  • Arterial Spurting (OR Gushing) Pattern - Bloodstain pattern(s) resulting from blood exiting the body under pressure from a breached artery.
  • Back Spatter - Blood directed back towards the source of energy or force that caused the spatter.
  • Expiratory Blood - Blood that is blown out of the nose, mouth, or a wound as a result of air pressure and/or air flow which is the propelling force.


Transfer/Contact bloodstains

A transfer or contact stain is produced when an object with blood comes in contact with an object or surface that does not have blood. It may be possible to discern the object that left the blood impression.
  • Wipe Pattern - A bloodstain pattern created when an object moves through an existing stain, removing and/or altering its appearance.
  • Swipe Pattern - The transfer of blood from a moving source onto an unstained surface. Direction of travel may be determined by the feathered edge.
As indicated above, there are other terms currently used in BPA and different ways of classifying bloodstain patterns. For example there is a debate over the misnomer of the LVIS, MVIS, and HVIS as it relates to the physical term "velocity". A sub-committee of the SWGSTAIN has been tasked with addressing the terminology issues and develop a taxonomy for bloodstain patterns.


"Velocity" impact stains

Contrary to what the name states, the terms low-, medium-, and high-velocity impact spatter do not describe the velocity of the blood droplets as they fly through the air. The variation in the "velocity" is meant to describe the amount of energy transferred to a blood source in order to create the stains. Velocity is a speed (m/s) with a direction. Often the terms force and energy are quoted in conjunction with the unit ft/s or m/s which is incorrect. Force is related to velocity and mass (N or 1 kg ·m·s−2). Energy (work) is related to the force exerted on an object (J or N·m or kg·m2·s−2). As indicated above, there has been great debate over these terms and their definitions. Below is one method of differentiating low-, medium-, and high-velocity impact spatter. The term "impact" should not be confused with the mechanism by which the spatter was created. It actually refers to the blood impacting onto an object or area with some degree of force.


Low velocity impact spatter

Low velocity impact spatter (LVIS) is generally produced when objects traveling less than 1.5 m/s come in contact with a blood source. The preponderance of stains is generally larger than 3 mm in diameter.


Medium velocity impact spatter

Medium velocity impact spatter (MVIS) is generally produced when objects traveling between 1.5 m/s and 7.5 m/s come in contact with a blood source. The preponderance of stains is generally between 1 mm and 3 mm in diameter. Mechanisms that could produce this type of pattern include blunt force trauma or cutting/stabbing actions.

[edit]High velocity impact spatter

High velocity impact spatter (HVIS) is generally produced when objects traveling greater than 30 m/s come in contact with a blood source. The preponderance of stains is generally smaller than 1 mm in diameter. This pattern often has a mist-like appearance. High velocity patterns may be created by gunshots or explosives, but may also be caused by industrial machinery, coughing, or sneezing.


Blood

Blood is a tissue that is circulated within the body to assist other parts of the body. This connective tissue has specialized cells that allow it to carry out its complex functions. For a healthy person, approximately 8% of their total weight is blood. For a 70-kg (154-lb.) individual, this equates to 5.6 L (12 US pints).


Biological considerations

Blood contains three components suspended within plasma. The three components are erythrocytes, leukocytes, and platelets.
  • Erythrocytes - also known as red blood cells, are transporters. The role of erythrocytes is to transport oxygen. To do this it produces great quantities of hemoglobin, which gives it the distinct red colour. Blood that has passed through the heart and been oxygenated (in the arteries) tends to have a brighter shade of red as opposed to blood that is returning to the heart (in the veins). There are about 30 trillion erythrocytes circulating in the human blood at any given time.
  • Leukocytes - also known as white blood cells, are defenders. The role of leukocytes is to defend the body against harmful bacteria and microorganisms. There are five different types of leukocytes all having different sizes, shapes, structures, and functions. Leukocytes fight infection and disease. There are about 430 billion leukocytes circulating in the human blood at any given time (~1 per 700 erythrocytes).
  • Platelets - are pieces of larger cells that have broken off in the bone marrow. These bits of cytoplasm are enclosed by a membrane and do not have a nucleus. They play a major role in haemostasis (control of bleeding) by plugging up a breach in a vessel.
Plasma is a yellowish fluid that carries the suspended erythrocytes, leukocytes, and platelets. It is composed of water (92%), proteins (7%), and other materials such as salts, waste, and hormones, among others. Plasma makes up about 55% of blood. The remaining 45% is blood cells and platelets. Because plasma is lighter than the blood cells and platelets, it can be easily separated. Plasma does not separate from blood cells in the body because it is in a constant state of agitation.


Physical considerations

In physics there are two continuous physical states of matter, solid and fluid. Once blood has left the body it behaves as a fluid and all physical laws apply.
  • Gravity - is acting on blood (without the body's influence) as soon as it exits the body. Given the right circumstances blood can act according to ballistic theory.
  • Viscosity - is the amount of internal friction in the fluid. It describes the resistance of a liquid to flow.
  • Surface tension - is the force that gives the ability to blood to maintain its shape. When two fluids are in contact with each other (blood and air) there are forces attracting all molecules to each other.


Blood spatter flight characteristics

Experiments with blood have shown that a drop of blood tends to form into a sphere in flight rather than the artistic teardrop shape. This is what one would expect of a fluid in freefall. The formation of the sphere is a result of surface tension that binds the molecules together.
This spherical shape of blood in flight is important for the calculation of the angle of impact (incidence) of blood spatter when it hits a surface. That angle will be used to determine the point from which the blood originated which is called the Point of Origin or more appropriately the Area of Origin.
A single spatter of blood is not enough to determine the Area of Origin at a crime scene. The determination of the angles of impact and placement of the Area of Origin should be based on the consideration of a number of stains and preferably stains from opposite sides of the pattern to create the means to triangulate.


Determining angles of impact

As mentioned earlier a blood droplet in freefall has the shape of a sphere. Should the droplet strike a surface and a well-formed stain is produced, an analyst can determine the angle at which this droplet struck the surface. This is based on the relationship between the length of the major axis, minor axis, and the angle of impact.
A well-formed stain is in the shape of an ellipse (see figure 1). Dr. Victor Balthazard, and later Dr. Herbert Leon MacDonell, realized the relationship of the width-length ratio of the ellipse was the function of the sine of the impact angle. Accurately measuring the stain will easily result in the calculation the impact angle.
Fig. 1 Upward moving bloodstain showing proper ellipse placement.
Fig. 2 Angles of Impact

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