Blood: Forensic Uses of Blood at a Crime
Scene
Blood
properties
Blood types
Bloodstain Pattern Analysis
What is Blood?
• Fluid circulating throughout the body
• Transports oxygen, electrolytes, nourishment,
hormones, vitamins and antibodies to tissues and
transports cellular waste to excretory organs
• Components of blood:
• Plasma-55% of the blood (straw-colored liquid
in which the blood cells are suspended)
• Red blood cells (anucleated – carry O2 and
CO2)
• White blood cells (nucleated – defense against
infection and disease)
• Platelets (cell fragments responsible for
clotting)
What is Blood?
• On average, blood accounts for 7% of total body
weight (about 10 pints)
• 5 to 6 liters of blood for males
• 4 to 5 liters of blood for females
• A 40 percent blood volume loss, internally or/and
externally, is required to produce irreversible
shock (death).
•In 1901, Dr. Landsteiner recognized that all
human blood was not the same and he worked out
the ABO classification system.
•In 1940 he discovered the rhesus factor (Rh) in
blood.
Microscopic
Views
Fish Blood
Bird Blood
Horse Blood
Frog Blood
Cat Blood
Dog Blood
Human Blood
Snake Blood
Blood Disorders: hemophilia and sickle cell
disease
Normal Human blood on left, human blood with
sickle cell anemia on right
Blood Types

There are 4 major groups based on the presence
of, and type of surface cell proteins:
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AB: have both A and B blood proteins and produce no
antibodies
A: have only A proteins and produces antibodies to B
B: Have only B proteins and produces antibodies to A
O: have no blood proteins can produce antibodies to
both A and B
There are also surface cell proteins called Rh
factors

Individuals either have these (Rh+) or lack them (Rh-)
Blood Types: Codominance and Multiple Alleles
In blood types, there are three forms that
your genes can come in – they can contain
information for making the A protein, or the
B protein, or neither protein
 Let I = a gene, and a superscript for the
form that gene comes in:

IA = the A protein
 IB = the B protein
 i = no protein

Genes
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You must have 2 copies of each gene, and the
combinations will determine the proteins you
make, and therefore your blood type:
One final point, the A and B alleles are
codominant (one is not dominant over the other),
but both are dominant over the i gene
IAIA or IAi = blood type A
IBIB or IBi = blood type B
IAIB = blood type AB
ii = blood type O
Set-up a Punnett Square to determine blood type
of offspring…
The Underlying Genetic Bases of Blood Types
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A blood will agglutinate in anti-A serum because it
carries B antibodies. B blood will agglutinate in
anti-B serum because it carries A antibodies.
Recall that blood types are determined by proteins
found on the surface of red blood cells
Proteins are made by information in your DNA
(genes)
Recall that you have 2 copies of each gene, and
their make-up will determine what kind of protein
you make
Blood type is determined, in part, by the ABO blood group
antigens present on red blood cells
Blood Types - Testing
Probabilities
41% of US citizens has type A blood
 10% have type B blood
 4% have type AB blood
 45% have type O blood
 85% have Rh+ blood
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So what is the probability of having B- blood?
 10/100
x 15/100 = 150/10,000 or 1 in 67
(so mathematically, Weber is not 1 in a million; only 1 in 67
based on blood type )
Forensic Use of Blood Types
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Serology is used to describe a broad scope of
laboratory tests that use specific antigen and
serum antibody reactions.
Although DNA analysis has replaced most
conventional serology tests, there is still some
useful information in the blood
About 80% of the population are secretors
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Secretors secrete their blood antigens into their tears,
sweat, semen and saliva
The presence of any of these substances permits
identification of the blood type of the suspect
Knowing the blood type allows you to rule out
suspects (called exclusionary evidence) rather than
identify them. Why?
Blood as individual evidence???
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Blood typing can be applied to a host of enzymes and
proteins that perform specific functions in the body. Their
presence or absence varies within the population. More
than 150 serum proteins and 250 cellular enzymes have
been isolated.
Therefore, it is possible to use blood typing as individual
evidence; however, it is not practical to achieve the
statistics required because of the time and techniques
involved. Also, most factors degrade with time.
ABO/Rh typing, and often another kind of typing called
MNS, are used as exclusionary tests in forensic science
and paternity testing.
Problem to try…
The typical population in the United States
shows and MNS distribution of M=30%,
N=27%, S=48%.
If a blood stain found at the scene of a crime
is found to be AB, N, Rh-, what are the
chances that a suspect would have this
combination of antigens? Is this good
enough to convince a jury?
Criminalists must be prepared to answer the
following questions when examining dried blood:

Is it blood?
Determination of blood is best made by a preliminary
color test (mostly due to a catalytic decomp of peroxides by
hemoglobin)
Hemastix strips or Hematest tablets: turns blue
Kastle-Meyer: turns bright pink
Luminol: produces light in a darkened area

What is the species of origin?
Preciptin test determines animal or human blood
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If it is human, how close can it be
associated with a particular individual?
Caution – Gives False Positives!
The Kastle-Meyer test detects the presence
of enzymes in the blood
 Other substances such as potatoes and
horseradish contain the enzyme peroxidase
which will also react with phenolpthalien
 Thus, a positive Kastle-Meyer test is not
definitive, and only indicates the possible
presence of blood
 Other tests like precipitin are needed to
verify the presumptive results
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Luminol
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Works in a similar fashion to phenolphthalein – except
detecting chemiluminescence instead of pink color.
The chief catalyst (esp. in old blood samples) is iron, not
catalase
False positives are possible: Cu, Fe, Co, bleach, plaster
walls
Can test large areas at once
Does not degrade DNA or blood antigens
Luminol
Blood Spatter Analysis
Most evidence at a crime scene such as
hairs, footwear, fingerprints and DNA is
interpreted to determine the identity of the
individuals involved
 In contrast, bloodstain pattern analysis is
used to determine what happened at a
crime scene, and the sequences of events
that took place
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Blood Spatter
Blood drops form different shapes and sizes
 Blood spatter analysis uses the shapes
and sizes to reconstruct the crime scene.
Blood – a Liquid
To a blood spatter analyst, blood is nothing
more than a liquid
 As such, it behaves in predictable ways
(fluid dynamics)
 The features that blood has that many other
liquids don’t are:

Permanence: when blood dries, it leaves a
visible residue
 Color
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Direction, Volume and Surface
Texture Affect Blood Patterns
Directionality: relating to or indicating the
direction a drop of blood has traveled from
its point of origin
 Spot size and shape: vary according to the
amount of blood, the manner in which it is
propelled from a source and the surface
texture the blood drop lands on
 In general, the harder and less porous a
surface, the less spatter results.

Blood Spatter
Blood drops fall as small
spheres – not as tear drops
– due to its surface tension
& cohesion
 The smaller the drop, the
more spherical it will be
 Under normal conditions,
blood will form drops of
uniform size – roughly 0.05
ml and 6 mm in diameter
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Surface and Blood Spatter
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The type of surface the blood strikes affects
the amount of resulting spatter,
including the size and appearance of the
blood drops.
Blood droplets that strike a hard smooth
surface, like a piece of glass, will have little
or no distortion around the edge.
Blood droplets that strike linoleum flooring
take on a slightly different appearance.
There may be distortion (scalloping) around
the edge of the blood droplets.
Surfaces such as wood or concrete are
distorted to a larger extent. Notice the
spines and secondary spatter present.
Single drop of blood falling from various heights (m) onto various surfaces
0.5
0.5
1
1
Height/Surface
2
2
3
3
smooth
floor
paper
towel
fabric
Blood Spatter – Size of Drop
Determining Distance Blood Falls
Drops form circle when hitting surface
 Size depends on speed of blood drop
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Blood Spatter
Determining Distance Blood Falls
 Faster drop = larger diameter (size)
 Higher distance = larger diameter

Due to air resistance, speed maxes out at
distances above about 7 feet
Blood Spatter
Determining Distance Blood Falls
 However, size of drop also depends on the
volume of the drop.

Volume depends on object blood originated
from (needle = small; bat = large)
Blood Spatter
 Since
the volume of blood in a drop is
generally unknown…
 It is not possible to establish with a
high degree of accuracy the distance
that a passive blood drop has fallen
 So, what can we tell from bloodstains?
Blood Spatter – Horizontal
Motion
Determining Direction of Blood
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When a drop hits at an angle other than 90º, it will form a
tear drop shape on the surface it hits
Narrow end of a blood drop will point in the direction of
travel.
Remember that a drop at 90º leaves a spherical drop
The steeper the angle, the longer and narrower the drop is
Direction of Blood
one exception!
tail points in
direction of
travel
Blood Spatter
Determining Direction of Blood
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The angle can be determined
mathematically.
Width/Length, then take the
inverse sin (sin-1).
This number is the impact
angle (90 = perpendicular to
surface; <10 at a sharp angle)
Blood Spatter
Determining Direction of Blood
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If more than one drop (from spatter) results,
the point of origin can be determined
Blood Spatter
Determining Direction of Blood
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This is a 2-dimensional point of origin.
It is possible to determine the 3-D point of origin
Blood Spatter
For each blood
drop, a string can
be guided back to
the point of origin.
Types of Spatter
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Passive spatter: Passive bloodstains are drops
created or formed by the force of gravity acting
alone (ex: blood dripping off of a knife)
Projected spatter: Projected bloodstains are
created when an exposed blood source is
subjected to an action or force, greater than the
force of gravity (internally or externally produced)
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Forward spatter from an exit wound; back spatter from
an entrance wound.
Spattered blood can:
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Help determine the location of the origin of the blood
source.
Help determine the mechanism which created the
pattern.
Passive spatter and transfer patterns
Types of spatter cont.
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Projected: types of patterns
Arterial gush-pattern shows large spurted stains
for each time that the heart pumps. Arterial
blood is bright red, while veinous blood is dark
red (oxygenated vs. deoxygenated)
 Cast-off-created when a blood-covered object
flings blood in an arc onto a nearby surface
 Low velocity (cast off spatter)
 Medium velocity (cast off spatter)
 High velocity spatter (gunshots)
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Types of Projected Blood Spatter
Spattered Blood = random distribution of
bloodstains that vary in size
 Amount of blood and amount of force affect
the size of blood spatter.
 Can result from gunshot, stabbing, beating
 In general, for higher impacts, the pattern is
more spread out and the individual stains
are smaller.
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Medium impact = beating
High impact = gunshot
Projected – Arterial Spray
Arterial Spurt / Gush: Bloodstain pattern(s)
resulting from blood exiting the body under
pressure from a breached artery
 Large volume projected from arterial pressure
Projected –
Cast-off
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Usually found on ceilings – formed by the upstroke
of a weapon containing blood (bat, crowbar etc)
Number indicates number of blows, plus one
When found on a horizontal surface, suggests
sideways motion. Blood is deposited horizontally
by the “backswings”
Cast off blood is usually traveling slowly – less
than 5 ft per second
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Consequently the drops are typically smaller than
passive spatter (13-22 mm) – usually about 6mm (¼”)
or less
Projected-Low Velocity
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Deposited at less than
5 feet per second
Usually about 4mm in
diameter
Usually is from a
source that is dripping
blood
Projected – Medium Velocity
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A bloodstain pattern caused by a
medium velocity impact or force
to a blood source.
These type of spatters are
normally smaller than those from
low-velocity droplets (1-4 mm or
less) and tend to come from
impacts with blunt or sharp objects
which distribute blood in all
directions from the source of
impact.
These can help determine the
point of origin.
Blood is typically traveling
between 5 -25 feet per second
Projected – High Velocity
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High velocity blood spatter occurs
when a strong, explosive force projects
blood in a aerosolized spray
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These patterns produce very small
droplets
The blood is in a mist, and as such,
does not have much horizontal
movement (less than 5ft)
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Usually produced by a gunshot
Blood is traveling 100 ft/sec or more
Blood spatter is typically <1 mm
Gunshots result in back spatter (where
bullet enters) and forward spatter (where
bullet exits)
Moral of the Story

Calculation using “straight-line” trajectory
(no gravity and no air resistance) predicts a
“launch” point higher than actual point.
 Calculations
that don’t account for gravity and
air resistance usually give you a result that is
twice that of the actual height – so you can
correct (roughly by dividing your calculations by
2

More accuracy requires a better model and
more specialized work.
Blood impression on pants, spatter on a wall, high velocity
droplet and a flake of dried blood taken off of fabric