bloodstain pattern analysis bloodstain pattern analysis bloodstain pattern analysis (bpa) is a discipline of forensic science that consists of the study and analysis of bloodstains at a known or suspected crime scene. this is done with the purpose of drawing inferences about the nature, timing and other details of the crime. it is used mostly to study homicide or other violent crimes in which blood is present and can help in crime scene reconstruction. since the late 1950s, bpa experts have claimed to be able to use biology, physics (fluid dynamics), 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. part of a series onforensic science physiological anthropology biology bloodstain pattern analysis dentistry dna phenotyping dna profiling entomology epidemiology limnology medicine palynology pathology podiatry toxicology social psychiatry psychology psychotherapy social work criminalistics accounting body identification chemistry colorimetry election forensics facial reconstruction fingerprint analysis firearm examination footwear evidence forensic arts profiling gloveprint analysis palmprint analysis questioned document examination vein matching forensic geophysics forensic geology digital forensics computer exams data analysis database study malware analysis mobile devices network analysis photography video analysis audio analysis related disciplines electrical engineering engineering fire investigation fire accelerant detection fractography linguistics materials engineering polymer engineering statistics traffic collision reconstruction related articles crime scene csi effect perry mason syndrome pollen calendar skid mark trace evidence use of dna inforensic entomology outline categorythe 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." 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 that 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." 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). a 1952 episode of the police procedural radio series dragnet made reference to bloodstain pattern analysis to reconstruct a shooting incident. 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). 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, following the laws of physics. if this leaking blood ever reaches the surface of the skin, it will be externalized and will likely 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. here are its three main physical properties: 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 shows how to determine area of convergence from blunt trauma blood spatter. examples of blood-spatter and droplet patterns diagram showing how to calculate the area of origin of a blood splatter a bloodstain can present itself differently depending on the situation and the material on which it appears, and bloodstains may be hard to examine on porous surfaces such as fabric, and may be distorted. bloodstain pattern analysts consider the angle of impact to determine its origin and the amount of force behind it; variations in external forces can cause satellite drops. a point of origin can be determined by finding what bloodstain analysts call the "area of convergence" for the blood droplets. to find this point of origin, the shape of the blood and the length are often taken into account and the stringing method is implemented. this method involves drawing lines from each blood splatter and finding the area in which all the blood intersect. additionally, the angle of impact as well as other external factors such as the material on which the blood falls can change the shape and size of the blood. the point of impact can change the shape of the bloodstain. bloodstains, instead of maintaining their original forms, may become elongated. in these cases, the blood may have a tail capable of indicating directionality. in order to find the angle of impact investigators measure the length and width of the blood droplet and use the formula s i n ( a ) = w i d t h / l e n g t h . the (a) representing angle of impact.how to calculate the point of origin of a blood spatter impact spatter cast-off blood stain pattern impact spatter is the most common bloodstain pattern type in a crime scene. it occurs when an object hits a source of blood. in impact blood spatter patterns, blood is often circular and not elongated. there are two types of impact spatter, back spatter and forward spatter. back spatter occurs when blood is projected back at an attacker, while forward spatter is blood that exits directly from the victim's wound and projects onto nearby surfaces. the speed of the weapon used in the attack can cause changes in the size of blood spatter. the speed of the attack is classified into high, medium and low velocity attacks. high-velocity spatter (e.g., gunshot wounds) create small-sized droplets. high-velocity spatter usually travels 100 feet per second and creates blood droplets sized 1 millimeter or less. medium-velocity spatter (e.g., blunt force trauma) is often made with a weapon and can create cast-off patterns. they are often made at between 5 and 25 feet/second the blood droplets ranging from 1 to 4 millimeters in length. low-velocity spatters are usually created just as a result of blood dripping from the individual (i.e., gravity). they can also be referred to as passive or gravity bloodstains, and are separated further into four categories: transfer/contact stains, flow artefacts, drop stains, and pooling. transfer stains occur when two surfaces come into contact and at least one has blood on it, and it includes swipe and wipe patterns, which can give information regarding sequence of movement in some cases. pooling occurs when the source of the bleeding remains static for a certain period of time, the blood continuously dripping in the same location and resulting in an important accumulation. if the individual who is actively bleeding moves while blood is dripping, the resulting pattern will allow for determination of direction and relative speed of movement at that time. cast-off patterns are associated with impact spatter, these patterns are created when the object that is used in the attack is swung and the blood on the object is swung onto a nearby surface. these patterns may be used to guess the direction of a weapon swing. in these cases, the length and the shape of the bloodstain patterns can help determine the speed of the swing.