Bloodstain pattern analysis

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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.^[1] 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.^[2]

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.^[3]

Grounded in principles of physics, biology, chemistry, and medicine, 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,^[4] 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.^[5] Ideally, BPA involves meticulous evaluation of pattern characteristics against objective criteria, followed by interpretation to aid crime scene reconstruction.^[5] However, the lack of discipline standards in methodology underscores the need for consistency and rigor in BPA practices.

Efforts by organizations like the Organization of Scientific Area Committees (OSAC) BPA Subcommittee aim to establish standards for training, terminology, quality assurance, and procedure validation within the discipline.^[6]

The validity of bloodstain pattern analysis has been questioned since the 1990s, and more recent studies cast significant doubt on its accuracy.^[7] 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."^[8][7] The report highlighted several incidents of blood spatter analysts overstating their qualifications and questioned the reliability of their methods.^[8][9] 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."^[10]