Bloodstain Pattern Analysis
| Author | Patrick Laturnus |
| Pages | 113-148 |
113
A. OVERVIEW OF THE DISCIPLINE: BLOODSTAIN PATTERN
ANALYSIS
In the courtroom, packed with strateg y and detail, there is great value
placed on physical evidence—in a sea of conflicting expert opinions and
complex legal jargon, physical evidence is perceptible, tangible. Blood-
stains are such evidence and, when the origins of a bloodstain can be
faithfully a nd scientifically explained, bloodstain pattern a nalysis can of-
fer key information about a case.
Bloodstain pattern analysis is the study of the shapes, sizes, and loca-
tions of bloodstains in an effort to interpret the physical events that gave
rise to their origin. If bloodstains are observed on a wall, for example, it is
possible to understand how they were created and where they came from
based on the shape, size, and location of the stains in relation to each
other and to the wall.1 This type of analysis can be useful when applied
to any bloodletting event but, in the criminal context, it is particularly
informative in cases such as a physical assault and murder.
Bloodstain pattern analysis (BPA) evidence has been accepted as ex-
pert testimony in Canadian courts since the 1878 case of R. v. McPherson and
Burke. On 26 July 1878, Ellen Bennett was raped and murdered. Dr. William
Hodgson Ellis, a professor at the School of Technology, testified at the trial
in Whitby, Ontario. In addition to the identification of blood, Dr. Hodgson
1 The surface on which the bloodstain is present is known as the substrate.
CHAPTER 6
Bloodstain Pattern Analysis
Patrick Laturnus
LEgAL ConTExT: CAiTLin PAkosH
114 6 Patrick Laturnus
Ellis testified to the significance of the number, size, and position of blood-
stains on the trousers of the accused. The beginning of BPA as it is known
today, however, can be traced to a 25 January 1977 second-degree murder
conviction, where Herb MacDonnell testified as a bloodstain analyst at the
trial of R. v. Sparrow.2 At this time, analysts received training based on experi-
ments and they began to exchange ideas that would advance the discipline
towards a more science-based approach. Subsequently, BPA became recog-
nized in the court system and trained individuals began to qualify as experts.
When there is a bloodletting crime, the initial crime scene investiga-
tor will assess the value of the evidence and call in a qualified analyst if
required. In Canada, these analysts are typically police officers with the
necessary training and experience. Just how the analysis is conducted has
changed over time. In the early days and up to the 1990s, much of the opin-
ion evidence was based on the experience of the analyst. With training and
experience, the differences between one bloodstain pattern and another
seemed obvious. The analyst could testify with confidence to the meaning
of the patterns and describe what events might have happened to create
the observed bloodstain pattern. For example, the observation of an impact
pattern indicates the location of the blood source being struck, while a pas-
sive blood trail could show the direction that the blood source was moving.
As this discipline evolved over time, analysts have moved towards the
scientific method of analysis. Although experience still plays a key role in
bloodstain pattern analysis, the analyst must now be able to explain the
underlying science when offering an opinion. Through the use of new tech-
nology and a more in-depth study of physics, bloodstain analysts are better
able to understand and explain the bloodstain patterns they have observed
and analyzed, particularly when compared to the early days of the discipline.
B. THE SCIENCE OF BLOODSTAIN PATTERN ANALYSIS
1) Basic Principles of Bloodstain Pattern Analysis: Key Terms
andConcepts
Bloodstain pattern analysis will provide information about bloodletting
events, but the validity of the discipline is based in its foundation in scien-
tific principles. The complexities inherent in assessing bloodstains, rely-
ing upon both science and experience, practically make this discipline a
science in its own right. The scientific terms used to describe an expert’s
2 For the appeal decision, see (1979), 55 C.C.C. (2d) 443 (Ont. C.A.).
Bloodstain Pattern Analysis 6 115
understanding of the evidence bridge the physical reality of bloodletting
scenes with the abstract nature of scientific theories. Science explains
why the evidence found in a bloodstain pattern is predictable and repro-
ducible. Two particular concepts, “surface tension” and “directionality”
underpin why and how an analyst will form certain conclusions. The sci-
entific principles most relevant to BPA are adopted from the disciplines of
mathematics, biology, and physics.
a) Surface Tension
When a liquid is exposed to air, the molecules within it hold on tightly to
each other. In doing so, the liquid becomes spherical because the liquid
naturally tends to minimize its surface area. This results in a “skin” form-
ing on the surface of the sphere. Surface tension allows certain species of
insects to walk on water. Of importance to BPA, surface tension also causes
blood to quickly form a sphere when exposed to air. That sphere will travel
intact through the air until it impacts a substrate. The resulting bloodstain
will provide evidence of its origin (i.e., “the three-dimensional location
from which spatter originated”).3
6.1. This diagram depicts how the sphere is formed in flight due to surface
tension. The arrows show the direction of air pressure as the droplet falls.
b) Directionality
When a blood droplet strikes a surface, it will result in a bloodstain. By ob-
serving the characteristics of that bloodstain, it is possible to determine what
direction it was moving at the time of deposition. Typically, the narrow or
pointed end of the bloodstain will point in the direction that it was moving.
If, for example, the bloodstain is pointed on its right side but rounded on its
left, then the bloodstain was moving from left to right, and vice versa.
3 Scientific Working Group on Bloodstain Pattern Analysis (SWGSTAIN), “Recom-
mended Terminology,” online: SWGSTAIN www.swgstain.org/resources.
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