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.
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.
Chemical considerations 
Upon exiting the body, bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) and hemichrome (HC). The fractions of HbO2, met-Hb and HC in a bloodstain can used for age determination of bloodstains and can be measured by Reflectance Spectroscopy 1.
- In vivo hemoglobin molecules are mainly present in two forms: one without oxygen, de-oxyhemoglobin (Hb) and one saturated with oxygen, oxy-hemoglobin (HbO2), both have iron in the Fe2+ state. HbO2 can auto-oxidize into met-Hb, which contains iron in the Fe3+ state. Met-Hb is incapable of binding oxygen. When met-Hb is formed in vivo it will be reduced back to Hb by reductase protein cytochrome b5, resulting in a small part (<1%) of met-Hb in a healthy in vivo situation.
- Outside the body hemoglobin saturates completely with oxygen in the ambient environment to HbO2. Due to a decreasing availability of cytochrome b5, necessary for reduction of met-Hb, the transition of HbO2 into met-Hb will, in contrast to inside the body, no longer be reversed. Once hemoglobin is autooxidized to met-Hb it will denature to hemichrome (HC), which is formed by an internal conformation change to the heme group.
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 acts 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 maintains the shape of a drop of liquid, such as blood. 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 an oscillating 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 that the width-length ratio of the ellipse is the sine of the impact angle. Accurate measurement of the stain thus allows easy calculation of the impact angle.
Angles of impact 
Because of the three-dimensional aspect of trajectories there are three angles of impact:
- alpha (α), the impact angle of the bloodstain path moving out from the surface (see figure 2 with alpha at the top by the stain).
- beta (β), the angle of the bloodstain path pivoting about the vertical (z) axis (see figure 2 with beta extended to the floor).
- gamma (γ), the angle of the bloodstain path measured from the true vertical (plumb) of the surface (see figure 2 showing plumb line and extended angle from stain).
All three angles are related through the following trigonometric equations:
- = length of ellipse (major axis)
- = width of ellipse (minor axis)
Accurately measuring the stain and calculating the angle of impact requires due diligence of the analyst. In the past analysts have used a variety of instruments. Methods currently used include:
- Viewing loupe with an embedded scale in 0.2 mm increments or better that is placed over the stain. The analyst then uses a scientific calculator or spreadsheet to complete the angle calculations.
- Bloodstain Pattern Analysis (BPA) software that superimposes an ellipse over a scaled close-up image of an individual bloodstain, then automatically calculates the angles of impact.
Using software produces a very accurate result that is measurable and reproducible.
Point and area of convergence 
To determine the point/area of convergence an analyst has to determine the path the blood droplets travelled. The tangential flight path of individual droplets can be determined by using the angle of impact and the offset angle of the resulting bloodstain. “Stringing” stains is a method of visualizing this. For the purpose of the point of convergence, only the top view of the flight paths is required. Note that this is a two-dimensional (2D) and not a three-dimensional (3D) intersection.
- The point of convergence is the intersection of two bloodstain paths, where the stains come from opposite sides of the impact pattern. (see figure 3)
- The area of convergence is the box formed by the intersection of several stains from opposite sides of the impact pattern. (see figure 4)
In the past, some analysts have drawn lines along the major axes of the stains and brought them to an area of convergence on the wall. Instead of using a top-down view, they used a front view. This provides a false point/area of convergence.
Area of origin 
The area of origin is the area in three-dimensional space where the blood source was located at the time of the bloodletting incident. The area of origin includes the area of convergence with a third dimension in the z direction. Since the z-axis is perpendicular to the floor, the area of origin has three dimensions and is a volume.
The term point of origin has also been accepted to mean the same thing. However, it has been argued, there are problems associated to this term. First, a blood source is not a point source. To produce a point source the mechanism would have to be fixed in three-dimensional space and have an aperture where only a single blood droplet is released at a time, with enough energy to create a pattern. This does not seem likely. Second, bodies are dynamic. Aside from the victim physically moving, skin is elastic and bones break. Once a force is applied to the body there will be an equal and opposite reaction to the force applied by the aggressor (Newton's third law of motion). Part of the force will move the blood source, even a millimetre, and change the origin while it is still producing blood. So the source becomes contained in a three-dimensional volume, or region.
As with the area of convergence, the area of origin is easily calculated by using BPA software. There are other longer, mathematical methods of determining the area or origin, one of which is the tangential method.
- Point (Area) of Origin - The common point (area) in three-dimensional space to which the trajectories of several blood drops can be retraced. (see figure 5)
Crime scene photography has some unique requirements. When there is a bloodletting scene, the basics are still required but special attention must be given to the bloodstains. The current means of documenting the scene include 35 mm (B&W, colour, and specialty film), digital cameras, and video (Hi-8, DV, and other formats). Each method has its pros and cons. Often the scene is documented using multiple methods. (Videography has been included here because it follows the same principles and provides crime scene images.)
There are three types of crime scene photos:
- Overall – wide-angle images (28–35 mm range) that capture the scene as it is. This type of image provides anyone who has not been in the scene a good overall layout.
- Mid-range – images taken with a normal lens (45–55 mm range) give greater detail than the overall shots. In the case of a bloodletting scene, the mid-range image could capture a single bloodstain pattern.
- Close-up – images taken with a macro lens giving the greatest amount of detail. For example, a medium velocity impact pattern can contain thousands of individual stains where there is a preponderance of small stains (1–3 mm in diameter) some of which require individual images.
Many times an analyst cannot attend a bloodletting scene, and must work from the crime scene images and notes of the person who attended. An appropriate sized scale should be in overall, mid-range, and close-up images. For overall images the scales should be parallel and perpendicular to the floor. This provides the analyst, and anyone else who looks at the images, a proper perspective on what they are observing. (Note: in some cases overall and mid-range images are taken with and without a scale.)
In popular culture 
- Sam Tyler, the time-travelling detective in episode 3 of the Emmy Award-winning BBC TV series Life on Mars, asks his colleague Chris Skelton to make a blood pattern analysis, quickly realizing by his puzzled expression that in 1973 such techniques are not known.
- Serial killer Dexter Morgan of the Dexter novels and Showtime series is a blood spatter analyst for the fictitious Miami Metro Police Department.
- Catherine Willows is a blood spatter analyst on the CBS series CSI: Crime Scene Investigation.
- Bevel, Tom; Gardner, Ross M. Bloodstain Pattern Analysis With an Introduction to Crimescene Reconstruction, 3rd Ed. CRC Press 2008
- James, Stuart H.; Kish, Paul Erwin; Sutton, T. Paulette (2005). Principles of Bloodstain Pattern Analysis (3rd, illustrated, revised ed.). Taylor and Francis/CRC Press. ISBN 0-8493-2014-3. Retrieved 2009-01-30.
- Hueske, Edward E., Shooting Incident Investigation/Reconstruction Training Manual, 2002
- IABPA (International Association of Bloodstain Pattern Analysts). Suggested IABPA Terminology List. Retrieved October 2005 from: http://www.iabpa.org/Terminology.pdf
- IABPA (International Association of Bloodstain Pattern Analysts). Suggested IABPA Terminology List. Retrieved October 2005 from: http://www.iabpa.org/RevEduc.pdf
- Bremmer et al, Biphasic Oxidation of Oxyhemoglobin in Bloodstains. Plos One 2011. http://plosone.org/article/info:doi/10.1371/journal.pone.0021845
- James, Stuart H, Eckert, William G. Interpretation of Bloodstain Evidence at Crime Scenes, 2nd Edition, CRC Press 1999.
- Solomon, Berg, Martin, & Villee. Biology, 3rd edition. Saunders College Publishing, Fort Worth, 1993.
- Sutton, Paulette T., Bloodstain Pattern Interpretation, Short Course Manual, University of Tennessee, Memphis TN 1998
- Vennard, John King. Elementary fluid mechanics. John Wiley & Sons, New York, 1982.