Bloodstain pattern analysis (BPA) involves the study and analysis of bloodstains. In forensic work, bloodstain pattern analysis can help investigators find out more about certain aspects of previous events that took place at a place that is known or suspected of being linked to a crime in a significant way.
The bloodstain pattern can also gives us an indication of what type of weapon that was used. A small sharp weapon such as a honed cut throat razor will create a very different bloodstain pattern than that created by a blunt mallet or a stabbing blade.
A common misconception about blood in flight is that it has a natural tendency to form a teardrop shape in flight. Through experiments with blood, it has been shown that a drop of blood actually tends to form a sphere in flight.
The formation is caused by surface tension, which is what holds the drop together. Surface tension gives fluids a tendency to acquire the least surface area possible.
So, why is the fact that blood drops in flight tend to be spherical important for bloodstain pattern analysts? Because it helps them make an accurate calculation of the angle of impact when they analyze blood that has hit a surface after being in flight. Knowing the angle of impact is important for determining from which point or area the blood originated.
To reliably determine the area of origin, the BP analyst needs to consider several blood drop stains, and these stains should ideally be from opposite sides of the pattern. Simply looking at a single drop isn’t enough to come to any reliable conclusion.
The area of origin is the three-dimensional space where the blood source was located when the blood began its journey.
It is important to remember that bodies tend to be dynamic. Not only can the source of blood be engaged in major movements, but there are also small movements to take into account, such as the elasticity of skin. When a force is applied to the blood source (e.g. when an attacker strikes the victim), an opposite reaction will be applied (see Newton’s third law of motion) and this will moved the blood source somewhat, changing the exact point of origin.
The string method is one of the oldest techniques for determining an space in which the bloodletting occurred, based on a bloodstain pattern. First, angle of impact and area of convergence needs to be determined. Then, the BP analyst will place their protractor at the blood stain location and project a string at the angle of impact in the direction of the area of convergence. Now, the string will show a general origin of the bloodletting.
Bloodstains at a suspected crime scene are typically photographed in three different ways, since each way has its pros and cons when its time to analyze the scene. Photographing the bloodstains is important for several reasons, and one of them is that a BP analyst might be unable to attend the scene personally and will have to relay on documentation, including photographs and videos, provided by others.
This is wide-angle images that capture a large part of a scene. By looking at these photographs, you will get a good idea about the general layout and how the bloodstains are located within this environment. Overall photographs are usually 28-35 mm range images. Overall photographs are not ideal for analyzing details.
Overall photographs of a crime scene are often taken parallel and perpendicular to the floor.
Mid-range photographs are typically taken with a normal lens in the 45-55 mm range. They provide greater detail than wide-angle images, but less detail than close-ups. Mid-range photographs are often used to capture a single bloodstain pattern, giving the analyst a good idea about what the overall pattern looks like, but without providing information about how it related to the rest of the scene.
Close-ups are taken with a macro lens and are useful when you need to see small details, such as stains that are just one or a few millimeters in diameter. An analyst might fore instance wish to take a closer look at some of the thousands of individual stains that together form a medium velocity impact pattern.
One of the earliest scientific studies of bloodstain patterns was made by Dr. Eduard Piotrowski of the University of Kraków in 1895. Even long before this study, bloodstain pattern analysis was sometimes used by law enforcement, but without much formal science to back it up. Dr. Piotrowski’s paper dealt with the formation, form, direction, and spreading of blood stains after blunt trauma to the head.
LeMoyne Snyder’s book “Homocide Investigation”, published in 1941, was widely used by forensic analysts of that era. It mentions bloodstain analysis, but only briefly. Examples of facts pointed out in the book are that bloodstains tend to fall in certain patterns based on the motion of an attacker and the victim, and that blood dries at a relatively predictable rate.
The International Association of Bloodstain Pattern Analysts (IABPA) was formed in 1983 in Corning, New York State, USA, under the guidance of Herbert MacDonell. Herbert MacDonell was the researcher who published the first modern treatise on bloodstain analysis: “Flight Characteristics and Stain Patterns of Human Blood” in 1971. The first formal bloodstain training course was given by him in Jackson, Mississippi, in 1973.
IABPA was founded by a group of blood stain analysts who wished to further the then emerging field of bloodstain pattern analysis. It was in the early 1980s that forensic BPA started to grow and become a fairly regular occurrence in violent crime investigations in the United States. One of the main objects f the IABPA was to promote the education of those involved with BPA and to help create standards for reporting, terminology, analysis and training.
A report released by the United States non-profit organization The National Academy of Sciences in 2009 highlighted several cases where blood splatter analysts had overstated their qualifications. The report also questions the reliability of BPA methods.
Just as with many other forms of forensic evidence, bloodstain pattern analysis used in the legal system rests on the analyst’s interpretation of ambiguity, but this can be difficult to know for juries. Various forms of bias can impact the analyst’s interpretation and there is for instance the risk of confirmation bias.
Another often overlooked fact is that not all blood is alike in humans, which in turn makes it difficult to actually obtain general facts based on conclusion drawn after carrying out a small set of experiments.
For those interested in learning more about the pitfalls of BPA, here are a few suggestions of cases to look into: