Blood Pattern Analysis – is it worth the paper it’s spattered on?

 There is a widely held view that Blood Pattern Analysis (BPA) is subjective and although this isn’t the case, to some extent we, that is forensic scientists, have only ourselves to blame for the confusion. Historically, forensic experts would be summoned to a crime scene to offer an opinion regarding the blood-letting events that had taken place and we would happily do so without necessarily demonstrating the findings on which our opinion was based. Who would want to see our working out anyway? The provision of an explanation to fit a particular set of observations can be useful in the investigative phase of a case but the explanation is unlikely to be exhaustive and at worst, without the appropriate caveats in place, it has the potential to be misleading. Recording incomplete observations (or not recording them at all) doesn’t allow other operators to compare and contrast ideas, re-evaluate findings, interrogate the thinking or establish if the interpretation was robust. This does not fit comfortably with all that we expect of science and it doesn’t help to build a database of transferable knowledge on which future thinking can be based. It also doesn’t help that up until fairly recently no-one had the time to publish their thinking and because the early development of ideas in BPA was necessarily driven by practitioners, not academics, there was less of a culture to formalise knowledge and conceptualisation. Although this has arguably resulted in a gap in the BPA literature, what is available is not insignificant. This paper aims to demystify BPA, outline how it can contribute to an investigation and dispel the myth that BPA incorporates an element of black magic.

BPA is an objective science underpinned by research that dates back to (at least) the late 19th century. Piotrowski’s work, the ‘Origin, shape, direction & distribution of the bloodstains following head wounds caused by blows’ (1895) forms the foundation for the observation-based interpretation that can be applied by scientists today. In this context, objective is used to infer concurring opinion derived independently by two (maybe more) individuals coming to the same conclusion about the same observation. This is the gold standard for BPA and it is a position that we strive to achieve through training and continued professional development, whilst being cognisant of the fact that the evidence is based first and foremost on human observation and taking steps to accommodate this.

As with most scientific disciplines, bloodstain pattern analysts use a bespoke taxonomy and terminology (; accessed August 2015) which is designed to foster international consistency. As with most jargon, scientists must work to demystify these terms during the course of their reporting, or whilst giving oral testimony, so that everyone else can understand what they are actually talking about.

The unique terminology is particularly evident in the term ‘spatter’, which is used to describe bloodstaining or patterns of bloodstains caused as a result of blood droplets landing on a surface. It is often the case that the gruesome details of crime are announced in terms of the spatter distribution that was left and it is true that droplets of blood, and the stains that they create, can be extremely probative. Impact spatter, bloodstaining generated as a result of a blow being delivered into a source of wet blood, can be indicative (but not exclusively so) of activities typically encountered in assaults for example.

Cast off bloodstaining (blood cast off an object wet with blood) can be equally informative and may appear as a drip trail or after having been thrown from an arcing weapon. Other forms of spatter include that which has been generated as a result of gunshot injuries (back and forward spatter) or projected from a damaged blood vessel, although the latter does not require an external force to create it but is distributed under the influence of blood pressure. Each can form distributions of blood that are characteristic and, in some cases, produce patterns that are unequivocal. This is particularly the case if blood has been mixed in some way before, or after, deposition for example with saliva, or attempts have been made to change its appearance, such as if cleaning has taken place.

This is the theory, and blood distribution scientists should have seen and preferably created these types of patterns (and others) for themselves in training, using animal-derived blood in order to establish an understanding of how blood behaves in controlled conditions. This knowledge prepares operators for the chaos of the real world. Case-related bloodstaining rarely demonstrates the crisp features that allow bloodstains to be classified categorically and usually indiscrete bloodstaining, blood that has been distorted and/or does not show sufficient diagnostic features, is encountered. The assessment of bloodstaining on clothing and footwear can be particularly challenging due to factors such as partial pattern capture, the dynamic nature of assaults, and folds and creases that are in place at the moment that blood is deposited. In comparison, the examination of blood at crime scenes can offer a more complete picture, providing patterns of blood that are amenable to reconstruction.

By understanding the influence that factors such as the physiological characteristics of blood, surface interaction, activities taking place during blood loss and the circumstances associated with a particular case, an expectation that blood could transfer to an individual given a set of alleged circumstances can be established. This is where BPA can add value to a case and critically can be used to evaluate the absence of blood, an area of BPA that is frequently underutilised. It is simply not the case that when blood is spilled the perpetrator will be heavily bloodstained, and in many cases those not involved in an assault can be stained more heavily than the assailant themselves.

BPA at its best can offer an insight into blood evidence that not only allows activities associated with blood loss to be determined but critically, from the perspective of the court, can be probative when evaluating the alternative scenarios that have been proposed: he kicked versus he did not kick, but walked through the blood of the injured party. In my experience, blood distribution evidence often plays second fiddle to the cogently argued gold standard of DNA. But, when the source of the blood (i.e. who’s blood it is) is not challenged (e.g. she did not punch but gave medical help, thereby accepting that blood from the injured party is present) ‘The Question’ emphatically changes from ‘who could the blood have come from?’ to ‘how did it get there?’ The latter is frequently overlooked or raised at the last minute, and can be posed for the first time to the individual standing in the witness box, which is unsatisfactory.

It is the scientist’s job to assist the court in putting this sometimes complex science into context and to present the strengths and limitations of blood findings in a clear and informative way using uncomplicated language. More importantly it is critical that any opinion is clear, justified and independently corroborated. We can only do this if ‘The Question’ is asked sufficiently early in the investigative process to be of use and to some extent this requires education and investment. Using DNA analysis to determine the potential donor of blood does not help in addressing how the bloodstaining may have been deposited.

Unlike a DNA or drug identification test, it can be difficult to put a unit price on BPA. Evaluative thinking is rarely time defined and this perceived open-endedness can be unpalatable when forensic budgets are under increased pressure. But imagine if the spend associated with DNA testing, for example, was instead used to determine how the bloodstaining had been deposited. This may be a relevant proposition if the person accepts that blood on their clothing came from a particular individual. Focusing a forensic strategy on BPA and not DNA requires bold decision making but this becomes easier if the protagonists have BPA knowledge and ultimately a BPA examination has the potential to generate an output which is more relevant to the court. In my opinion, every forensic operator who searches for, examines, or recovers samples of blood should have a level of BPA training which will support the early identification of relevant staining and the appropriate use of targeted testing. This is certainly not a short-term solution but if we can change our perception of BPA, rather than asserting that we can’t afford to do it, we will encourage the view that, in the context of blood cases, we simply cannot afford not to.

Case example:

An individual was charged with assault, namely kicking and stamping upon the complainant. The injured party suffered a number of injuries including to his face, which bled. Bloodstaining was found on the footwear of the alleged assailant and a sample of blood, from the largest stain that was present, was recovered for DNA analysis. The sample generated a full DNA profile, which matched that of the victim and a DNA-based report was submitted to the court. In interview with police, the suspect said that he had been out drinking with his friends, when someone from the group started to argue with the victim and a fight took place. The suspect was nearby at the time and he gave a very clear eye-witness account, but denied being involved in the attack. He said that he did not kick or stamp upon the victim’s head. As part of the defence review, the defendant’s footwear was re-examined. On the sole of his training shoe, within the recesses of the tread, a series of small discrete stains were observed which were associated with small runs or ‘tails’ of blood. This staining was indicative of percussive staining. Percussive staining can be formed when an item that is wet with blood comes to an abrupt stop. The characteristics of percussive staining can be further used to infer the direction in which the bloodstained item was moving at the moment at which it came to an abrupt stop and in this case, were suggestive of the training shoe having been moving towards the toe. In my experience, any movement must stop with considerable force in order for stains of this type to be generated. Walking and running for example rarely result in the production of percussive staining unless considerable forces are involved and a kicking action is more likely to cause staining of this type. In the context of the case, the presence of this form of staining on the footwear of the defendant was arguably significant and had it have been identified during the initial screening examinations, could have been helped CPS in reviewing the file.

Jo Millington – Senior Forensic Scientist, ArroGen Forensics Ltd

 ArroGen Forensics Ltd, Unit 12, The Quadrangle, Grove Technology Park, Wantage, Oxfordshire, OX12 9FA


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