Chapter 10: Bloodstain Pattern Analysis

Number of Blows

Often when an individual is struck with an instrument, blood is shed. If the individual is continually struck, the object doing the striking will begin to pick up blood and cast it off onto a surface. With each strike and resulting back swing a cast off pattern can be produced. By examining the number of these patterns, one can deduce a general number of strikes administered to the victim. However, one strike must always be added to this number as the initial blow is the one which produced the blood.

Direction

The directionality of a bloodstain or pattern indicates the direction the blood was traveling when it impacted the target surface. The direction of a blood drop can usually be established from the geometric shape of its bloodstain.In addition to interpreting bloodstain patterns, the technician is sometimes asked to make other determinations about the bloodstains which could have an effect on the outcome of the investigation.

Surface Tension

The force that pulls the surface molecules towards the interior of a fluid and decreases the surface area and cause the liquid to resist penetration.

Area of Origin

The location in three-dimensional space from which blood that produced a bloodstain originated; the location of the area of convergence and the angle of impact for each bloodstain is used to approximate this area.

Skeletonization

The process by which the edges of a stain dry to the surface in a specific period of time (dependent on environmental and surface conditions); skeletonization remains apparent even after the rest of the bloodstain has been disturbed from its original position.

Viscosity

The resistance to change of form or flow due to the mutual attraction of the molecules to each other.

Angle of Impact

When a drop of blood strikes a surface, the shape produced by that stain can be examined and the internal angle or impact angle between the direction of the blood drop and the plane of the surface it strikes can be determined. A mathematical relationship exists between the angle of impact of a blood drop and the width and length of the resultant bloodstain.

Drying Time

When blood is exposed to an external environment the drying process is initiated. The drying time of blood is a function of the bloodstain size and volume, the nature of the target surface and the influences of effects of the environment. When estimating drying times associated with case work, they should be very conservative. Only through comparative experiments under similar conditions can one come to a determination of the drying time of particular bloodstains.

Volume

When held together by the forces described above and only acted upon by forces of gravity, a single blood drop will have a standard volume of 0.05 ml.

Transfer Pattern

a bloodstain pattern created when a surface that carries wet blood comes in contact with a second surface; recognizable imprints of all or a portion of the original surface or the direction of movement may be observed

Flow

a bloodstain pattern formed by the movement of small or large amounts of blood as a result of gravity's pull

Expirated Blood Pattern

a pattern created by blood that is expelled out of the nose, mouth, or respiratory system as a result of air pressure and/or airflow

Satellite Spatter

blood spatter around parent stain, with blood droplets whose pointed ends face against the direction of travel

Medium Velocity -

produced on a surface with a force between approximately 5 to 25 feet per second-Elliptical in shape with a diameter between 1 mm to 1 cm

High Velocity

produced on a surface with a force of approximately 100 feet or greater per second-Round in shape with a diameter of 1 mm or less

Low Velocity

produced on a surface with a force of approximately 5 feet or less per second-Round in shape with a diameter of approximately 1 cm or larger

Impact Spatter

A bloodstain pattern produced when an object makes forceful contact with a source of blood, projecting droplets of blood outward from the source.

Cast off

A bloodstain pattern that is created when blood is flung from a blood-bearing object in motion onto a surface.

Medium-velocity Spatter

An impact spatter pattern created by a force traveling at 5 to 25 feet per second and producing drops with diameters between 1 and 4 millimeters.

Sequencing

Considerable activity, whether simple movement by an injured person or in a violent struggle where one or both participants are bleeding, will most often result in several overlapping bloodstain patterns. A study of these patterns can lead an investigator to determine the order of occurrence of events.

Low-velocity Spatter

An impact spatter pattern created by a force traveling at 5 feet per second or less and producing drops with diameters greater than 4 millimeters.

Arterial Spray

A characteristic bloodstain pattern containing spurts that resulted from blood exiting under pressure from an arterial injury.

Drop Trail Pattern

A pattern of bloodstains formed by the dripping of blood off a moving surface or person in a recognizable pathway separate from other patterns

Void

An area within a deposited spatter pattern that is clear of spatter, caused by an object or person blocking the area at the time of the spatter's deposition.

High-velocity Spatter

An impact spatter pattern created by a force traveling at 100 feet per second or faster and producing drops with diameters less than 1 millimeter.

Aging

As bloodstains increase in age, they progress through a series of color changes from red to reddish brown to green and eventually to dark brown and black. This change in color is attributable to alterations in the hemoglobin of the blood. A particular environment and especially the presence of bacteria and other microorganisms will affect the sequence and duration of color changes in bloodstains. Therefore, estimates of the age of bloodstains based upon their color at the scene should be avoided.

Back Spatter

Blood directed back toward the source of the force that caused the spatter.

Forward Spatter

Blood that travels away from the source in the same direction as the force that cause the spatter.

Relative to the reconstruction of a crime scene may provide information to the follow-up investigator to include:

Substantiate or discredit statements of witnesses, victims, and/or suspects Assist in identifying and collecting probative evidence Lead to additional crime scenes Assist in the reconstruction of events of the crime

Angle of Impact

The angle of the source of the blood to the surface where it was deposited. It can be estimated from the width-to-length ratio of the stain to help determine the position of the victim or the weapon at the time when the bleeding wound was inflicted.

Area of Convergence

The area on a two-dimensional plane where lines traced through the long axis of several individual bloodstains meet. This approximates the two-dimensional place from which the bloodstains were projected.