Bloodstain pattern analysis (BPA) is a forensic discipline focused on analyzing bloodstains left at known, or suspected crime scenes through visual pattern recognition and physics-based assessments. This is done with the purpose of drawing inferences about the nature, timing and other details of the crime.A Simplified Guide To Bloodstain Pattern Analysis, The National Forensic Science Technology Center (NFSTC), Florida International University At its core, BPA revolves around recognizing and categorizing bloodstain patterns, a task essential for reconstructing events in crimes or accidents, verifying statements made during investigations, resolving uncertainties about involvement in a crime, identifying areas with a high likelihood of offender movement for prioritized DNA sampling, and discerning between homicides, suicides, and accidents. Not to be confused with haemotaphonomy.
Since the late 1950s, BPA experts have claimed to be able to use biology, physics, and mathematical calculations to reconstruct with accuracy events at a crime scene, and these claims have been accepted by the criminal justice system in the US. Bloodstain pattern analysts use a variety of different classification methods. The most common classification method was created by S. James, P. Kish, and P. Sutton, and it divides bloodstains into three categories: passive, spatter, and altered.
Despite its importance, classifying bloodstain patterns poses challenges due to the absence of a universally accepted methodology and the natural uncertainty in interpreting such patterns. Current classification methods often describe pattern types based on their formation mechanisms rather than observable characteristics, complicating the analysis process. Ideally, BPA involves meticulous evaluation of pattern characteristics against objective criteria, followed by interpretation to aid crime scene reconstruction. However, the lack of discipline standards in methodology underscores the need for consistency and rigor in BPA practices.
The validity of bloodstain pattern analysis has been questioned since the 1990s, and more recent studies cast significant doubt on its accuracy. A comprehensive 2009 National Academy of Sciences report concluded that "the uncertainties associated with bloodstain pattern analysis are enormous" and that purported bloodstain pattern experts' opinions are "more subjective than scientific".National Research Council. Strengthening Forensic Science in the United States: A Path Forward. Washington, DC: The National Academies Press, 2009. Archived The report highlighted several incidents of blood spatter analysts overstating their qualifications and questioned the reliability of their methods. In 2021, the largest-to-date study on the accuracy of BPA was published, with results "show[ing] that [BPA conclusions] were often erroneous and often contradicted other analysts."
History
Early history Bloodstain pattern analysis has been used informally for centuries, but the first modern study of blood stains was in 1895. Dr. Eduard Piotrowski of the University of Kraków published a paper titled "On the formation, form, direction, and spreading of blood stains after blunt trauma to the head."Eduard Piotrowski, Ueber Entstehung, Form, Richtung und Ausbreitung der Blutspuren nach Hiebwunden des Kopfes [On the formation, form, direction, and spreading of blood stains after blunt trauma to the head] (Vienna, Austria: 1895). He conducted his first experiments on rabbits. A number of publications describing various aspects of blood stains were published, but his publication did not lead to a systematic analysis. LeMoyne Snyder's widely used book Homicide Investigation (first published in 1941 and updated occasionally through at least the 1970s) also briefly mentioned details that later bloodstain experts would expand upon (e.g., that blood dries at a relatively predictable rate; that arterial blood is a brighter red color than other blood; that bloodstains tend to fall in certain patterns based on the motion of an attacker and victim).Snyder, LeMoyne (1971). Homicide Investigation: Practical Information for Coroners, Police Officers, and Other Investigators. Charles C. Thompson Publishers, 3rd Edition A 1952 episode of the police procedural radio series Dragnet made reference to bloodstain pattern analysis to reconstruct a shooting incident."Judging from the bloodstains found on the furniture and rug in the living room, and on the front steps of the cottage, Radford had first been shot while he was in the living room..." Quote starts at about 12 minutes and 17 seconds into the episode. "The Big Streetcar", April 3, 1952; no script writer identified.
Acceptance as valid evidence in United States courts Between 1880 and 1957, courts in Michigan, Mississippi, Ohio, and California rejected expert testimony for bloodspatter analysis, generally holding that it added nothing to the jurors' own evaluations of bloodstains submitted as evidence. In 1957, the California Supreme Court became the first American court to accept expert testimony examining bloodstains, accepting as evidence the testimony of Paul L. Kirk, a professor of biochemistry and criminalistics. He would also testify in the Sam Sheppard case in 1966, when the wife of an osteopathic physician was beaten to death in her home, interpreting bloodspatter evidence as proof that the murderer was left-handed (Sheppard was right-handed). However, bloodstain pattern analysis would not begin to enter wide use until it was promoted by Herbert Leon MacDonell. MacDonell researched bloodstains with a grant from the United States Department of Justice, and which also published his research in the book "Flight Characteristics and Stain Patterns of Human Blood" (1971). The basis for MacDonell's alleged expertise was a set of experiments he conducted by extracting his own blood and then slinging it around in the basement of his house in Corning, New York. MacDonell testified in court on multiple occasions as an expert of bloodstain analysis, and the legal precedent set by these cases led to its widespread use in American courts, although as early as 1980 some judges expressed strong doubts about its reliability, and it was not always accepted as evidence, especially in states with no prior rulings that relied on such evidence.
The first formal bloodstain training course was given by MacDonell in 1973 in Jackson, Mississippi. MacDonell taught workshops on how to conduct bloodstain analysis, and the newly trained bloodstain analysts, who often had received as little as 40 hours of instruction, in turn would give expert testimony in court cases. In 1983, the International Association of Bloodstain Pattern Analysts was founded by a group of blood stain analysts to help develop the emerging field of bloodstain pattern analysis.
Further investigation into its admissibility as evidence Starting in 1995, court cases where bloodstain analysts disagreed with each other raised concerns of the discipline's prediction's admissibility as evidence in court. In 2009, the National Academy of Sciences published an examination of forensic methods used in United States courts which harshly criticized both bloodstain pattern analysis and the credentials of the majority of the analysts and experts in the field. Judges have largely ignored the report's findings and continue to accept bloodstain pattern analysis as expert evidence.
In 2013 Daniel Attinger, a fluid dynamics researcher at Columbia University, published a paper on bloodstain pattern analysis in Forensic Science International, finding that many of the central hypotheses of bloodstain analysis remain untested, and that existing analysts often made incorrect assumptions or other errors in their analyses. The paper also proposed fluid dynamics as a theoretical framework for solving these problems, and Attinger has continued to publish several papers exploring these concepts (as have other scientists as well). However, these papers are largely theoretical, and have had little impact on the use of bloodstain analysis in courts.
The physics of blood
Principles Blood is composed of three components suspended in plasma: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).
When a blood vessel is damaged, the blood starts moving outside the circulatory system. If this leaking blood reaches the surface of the skin, it will be externalized and may come into contact with the person's surroundings. Some of the most common interactions are:
Gravity An internal source applying pressure on the blood (expiration, blood pressure, etc.) External force applied at the source of the bleeding (e.g., being hit with a weapon) Transfer or alteration resulting from two surfaces coming into contact (swipe, wipe, etc.)
Since blood is 55% plasma (and 45% solid components suspended in plasma), it is an aqueous liquid and behaves as such. More specifically, it is a shear-thinning, non-Newtonian fluid, which has the following behavior with respect to three properties of liquids:
Viscosity (denoted by μ) describes the relation between force applied on the fluid and the rate at which the fluid particles can be separated from each other, or deformed. If a fluid has a high viscosity, it will not flow as easily as a fluid with a lower viscosity. Because this relates directly to the strength of the attraction between the fluid molecules, viscosity of blood decreases with increasing temperature and decreasing hematocrit. It also lowers with a higher shear, which is why it is classified as a shear-thinning, non-Newtonian fluid. This is why blood appears more watery when it hits a surface than right after it leaves the body. The closer the shear rate is to zero, the more viscous the blood is. Some other factors can also influence blood viscosity. Surface tension (denoted by σ ) describes the energy required to change the shape of a fluid when it comes into contact with another fluid in which it is not miscible (e.g., blood and air). This plays a big role in the formation of spherical blood droplets as they fall, as well as the way they react when they hit a surface. Surface tension can also be affected by outside factors such as temperature and the presence of chemicals in the blood. Density (denoted by d) represents a fluid's mass per unit volume, which allows for comparison between substances. The reference for comparison is usually water since it has a density of 1 g/cm3.
Bloodstain types