What is bloodstain pattern analysis (BPA)?

Bloodstain pattern analysis, (also referred to as blood spatter analysis and  blood splatter analysis) is an in depth evaluation of the bloodstain patterns found in a given scene.  To reach a conclusion the analyst considers a number of factors related to each pattern, including but not limited to: number dispersion, shape, size, volume orientation and location.  The product of this analysis are defined events that occurred during the incident; that is the bloodstain pattern defines the basic nature of the action that created it. 

What do you mean by basic nature?

Blood as a colloidal fluid is affected as all fluids are by associated physical laws (e.g. the effect of air resistance, gravity, surface tension). If we expose a mass of blood to a basic event (e.g. striking it, streaming it under pressure) the resulting pattern will demonstrate particular characteristics.  Thus bloodstain patterns are reproducible phenomena. Although various classifications systems exist to explain the resulting patterns, within all of these systems are basic pattern types. In one form or another, these will include:

• Disruption of blood at a point source (radiating spatter patterns)
• Dispersion of blood over time and space from an object in motion (castoff patterns)
• Dispersion of blood from a point source as a stream under pressure (projected patterns)
• Gravity induced patterns (drip patterns)
• Stains resulting from contact (contact/transfer patterns)
• Volume patterns (pools, flows) 

What is the goal of the bloodstain pattern analyst?

The goal of the bloodstain pattern analyst is to recognize this basic nature of the stain through these physical characteristics, properly classify the pattern and then to associate the pattern back to a source event within the context of the scene.  This final step is a function of associating the pattern to a specific event that occurred during the incident in question.  For instance:

In a scene we might find a radiating pattern of very fine spatter deposited on a surface in close proximity to a bullet defect.  The autopsy information advises us that the victim received a perforating gunshot wound.  DNA testing verifies the blood is that of the victim. Using BPA the pattern would be recognized as an impact spatter pattern.  The small (sub-millimeter) size of the stains in the pattern might suggest explosive force and thus the analyst could functionally opine that the pattern was produced by the gunshot event.

Can BPA always associate a pattern to a specific source event?

No. Bloodstain pattern analysis is class characteristic type evidence.  This ability to associate a specific pattern to a specific source event is the goal of BPA, but it is not always feasible. We can generally identify the basic type of pattern, but it is only through scene context that we can ultimately associate it to a specific event.  It is not uncommon that the analyst will recognize that a particular type of pattern is present, but not be able to limit it to a specific event. For instance:   

      In a case involving multiple blunt trauma blows delivered by both hands and a weapon, the analyst would certainly recognize the presence of multiple impact spatter patterns. But it is unlikely based only on the spatter patterns themselves that the analyst could associate one pattern to a particular blow by a particular weapon. 

Why do so many systems of classification exist in bloodstain pattern analysis?

In BPA there are various systems of classification. Each developed in its own right and each has value and function. Over time different professional groups and different teachers adopted different systems. Although on first glance it might seem as if they are disparate systems; as previously discussed inherent in all of these systems are the basic pattern types. The systems simply start from different initial perspectives. Examples of different classification systems include:

LVIS, MVIS, HVIS. This system concentrates on the idea of spatter size as a principle distinction.  Although well known, for various reasons associated to misuse and misunderstanding, this system is being replaced by other classification systems.

Passive/Dynamic.  This system is mechanistic and starts with the nature of the underlying event. Dynamic patterns are those associated to disruption mechanisms (e.g. impact, castoff, projected).  Passive patterns are contact stains and patterns produced by gravity.

Spatter/Non-Spatter.  This system is taxonomic in nature. The principle starting point is whether the pattern is made up of droplets that have been in free flight (small stains circular or elliptical in nature).  Spatter groups include impact spatter, arterial spatter, castoff stains, and drips.  The non-spatter include pools, flows and stains produced by contact.

Is BPA a science?

The answer to that question is likely one of opinion. Personally I answer that question with a no.  BPA is a technical discipline based on science.  BPA however must follow scientific method.

What is the methodology of BPA?

Different analysts may represent their methodology in different fashions. I describe it as seven steps.  Following scientific method, each step asks and answers specific questions.  Of course the ultimate goal is to try and associate a pattern back to a specific source event.  As I teach methodology these steps include:

• Assess and familiarize yourself with the scene.
• Identify discrete patterns.
• Classify the patterns.
• Consider directionality and motion.
• Consider points of convergence and area of origin if necessary.
• Evaluate interrelationships between patterns and other objects.
• Evaluate and identify viable source events, if possible.