Bloodstains have long occupied an important place in criminal investigations. At the scene of a violent crime, the position, shape and distribution of blood can appear to offer investigators a visual record of what happened. Bloodstain pattern analysis (BPA) has consequently been used to help reconstruct events, including the possible direction of impact, the movement of individuals and the manner in which blood came to be deposited.

Yet the apparent precision of bloodstain evidence can be misleading. Although the behaviour of fluids is governed by established principles of physics, interpreting a complex collection of bloodstains and translating those observations into a reconstruction of a criminal event can involve substantial professional judgement.

What Can Bloodstain Pattern Analysis Tell Investigators?

Bloodstain pattern analysts examine the location, shape, size and distribution of blood deposits. In appropriate circumstances, individual stains can provide information about directionality and the angle at which a droplet struck a surface.

More broadly, analysts may seek to distinguish between passive blood deposits, transfers and projected or otherwise distributed patterns. From those observations, opinions may be offered concerning how injuries occurred, the relative positions of individuals and the sequence of events.

The difficulty arises when relatively limited physical observations are used to construct a highly specific account of a crime. A bloodstain may provide evidence about how it was deposited without necessarily providing a complete explanation of why the wider pattern exists.

The Problem of Subjectivity

The reliability of BPA has attracted considerable scrutiny, particularly since the National Research Council’s 2009 report, Strengthening Forensic Science in the United States: A Path Forward. The report highlighted the significant uncertainties associated with bloodstain pattern analysis and questioned whether aspects of expert interpretation were more subjective than scientific.

Subsequent research has provided further reason for caution.

A major study involving 75 practising bloodstain pattern analysts examined 192 bloodstain patterns and generated more than 33,000 responses. Where the causes of the stains were known, approximately 11.2% of responses were erroneous. The researchers also found that analysts’ conclusions contradicted one another in approximately 7.8% of responses.

These figures do not mean that every bloodstain analysis is wrong, nor do they establish that BPA has no evidential value. They do, however, demonstrate that conclusions can vary between trained practitioners and that the limitations of the methodology need to be recognised.

Why Can Analysts Reach Different Conclusions?

One important issue is terminology. Where a forensic discipline lacks consistently applied definitions and classification standards, different analysts may interpret or describe the same pattern differently.

The physical surface is another important consideration. Blood behaves differently when it strikes glass, painted surfaces, clothing, untreated materials or different types of fabric. Absorption, spreading and the microscopic structure of a material can alter the appearance of an original stain.

Recent research into bloodstains on cotton fabrics illustrates this problem. Researchers found that the structure and weave of the fabric could affect the resulting stain and the information that could reasonably be extracted from it. In particular, features sometimes described as thin ‘fingers’ extending from a stain were found to vary with the velocity at which blood struck certain cotton surfaces.

Such research is potentially valuable precisely because it demonstrates that apparently straightforward bloodstain evidence can be influenced by numerous physical variables.

From Crime Scene to Courtroom

The history of BPA also illustrates how forensic techniques can acquire legal acceptance before their limitations are fully understood.

Herbert MacDonell became one of the most influential early figures in modern bloodstain pattern analysis in the United States. His experiments and courtroom demonstrations helped establish the technique as a form of forensic evidence. Over time, courts in several American states accepted testimony based upon BPA, with later decisions sometimes referring to earlier judicial acceptance when determining whether the evidence should be admitted.

This created a form of legal momentum: once courts had accepted the technique, subsequent courts could rely upon those earlier decisions when considering its admissibility.

The subsequent scientific scrutiny of BPA therefore raises an important distinction between admissibility and reliability. The fact that a court permits an expert to give evidence does not necessarily mean that every conclusion offered by that expert is scientifically established.

Science, Evidence and Expert Opinion

There is an important difference between measuring a physical phenomenon and interpreting what that phenomenon means.

The mathematics used to calculate the angle of impact of an individual blood droplet can be grounded in established physical principles. The difficulty becomes considerably greater when numerous stains are examined together and an analyst attempts to infer a detailed sequence of events.

That distinction matters in criminal proceedings. Expert evidence is intended to assist a court with matters requiring specialised knowledge, but the ultimate question of guilt remains for the court to determine. A forensic opinion should therefore be examined alongside the underlying observations, methodology, assumptions and limitations.

A More Cautious Approach to Forensic Evidence

BPA should not necessarily be dismissed simply because studies have identified errors and inconsistencies. Blood is a physical substance and its behaviour can provide genuine forensic information. The more significant question is how far that information can safely be taken.

The evidence increasingly supports a cautious distinction between what a bloodstain can objectively demonstrate and what an analyst believes may have happened.

For investigators, lawyers and courts, that distinction is critical. A stain may provide one piece of evidence within a much larger evidential picture, but turning a collection of stains into a definitive narrative requires assumptions that may not always be justified.

The continuing development of experimental research, improved terminology, standardised methodology and quantitative techniques could strengthen the discipline. Until then, bloodstain pattern evidence is best approached with an appreciation of both its physical foundations and its acknowledged limitations.

In criminal litigation, the central question should not simply be whether bloodstain pattern analysis can produce an explanation. It should be whether the particular conclusion being advanced is sufficiently supported by the underlying evidence, methodology and scientific knowledge to justify the level of certainty being placed upon it.