Blood Spatter Interpretation

A study of what the impact pattern can tell us about the crime…

• Blood is frequently shed during crimes of violence.  It is often possible to reconstruct one or more possibilities of what may have occurred to produce certain bloodstain patterns. 

• Usually, it is much more important to establish what mechanisms could not have occurred to produce a bloodstain pattern.  

• To achieve this it is essential that all available bloodstains be examined, either at the scene of the crime or from good quality photographs. 

• Their overall size, shape and distribution must be examined overall and not just in isolated areas.

Bloodstain Pattern Interpretation

• It involves reconstructing the events that must have happened to produce the bleeding.

• It is not something that most law enforcement officials can do--it usually requires a specialist.

Characterization of Stains

• The first step when dealing with blood in any aspect of forensics/criminology is to know the characterization of bloodstains.

Three Major Questions

• Is it blood? • From what species did the

blood come from? • If the blood is of human origin,

how closely can it be associated to a particular individual?

If it’s human blood…

• The pattern should be investigated– Location of the pattern– The way the blood has been

spattered– The way the pattern looks is

extremely important

Impact Angles are Important

o It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion of the stain.

o The shape of the stain tends to change depending upon the angle of impact which caused the stain. For example, the more the angle decreases, the more the stain is less circular and more long.

Surface Texture

o Surface texture is one of the key components in determining spatter type.

o The harder the surface is, the less spatter will result. It is therefore extremely important to duplicate the surface in a controlled test.

Surface Impact Patterns

o When a droplet of blood hits a surface which is hard as well as smooth, the blood usually breaks apart upon impact. This in turn causes smaller droplets.

o The smaller droplets will continue to move in the same direction as the original droplet.

Determination of Angle of Blood

The drop on the right fell 42 inches at an angle of 60-degrees on to hard smooth cardboard, while the drop on the left fell the same 42 inches and on to the same surface, but at a 10-degree angle.

Influences of Surface Texture

The blood drop at the right struck a plastic wall tile falling again from 42 inches. On the left, the drop fell from the same distance on to a piece of heavy, unusual textured wallpaper.

Impact on Hard Surface

This drop of blood was tested traveling 4 feet per second, on to a hard cardboard surface. It was traveling from right to left at an angle of 56-degrees. Although not clear in the example, the test showed the smaller drops aimed toward the large one.

General Rules to Consider

• The smaller the size of bloodspatters, the greater the energy required to produce them.  Low, medium, and high velocity impact spatter may be identified by their respective sizes but exceptions must also one considered.

• Before a drop of blood can fall, absent any other form of applied energy, gravitational attraction acting on blood must exceed its surface tension.

• Diameter of a large bloodstain will be of little or no value in estimating the distance a drop of blood has fallen prior to impact.

• When considering the shape of a bloodstain for use in calculating its angle of impact, only a sharp, well-defined bloodstain should be used for measuring its width and length.

• Directionality of a blood drop while in flight is usually obvious from the geometry of its resulting bloodstain. 

• The pointed end indicates the direction of travel prior to impact on a surface. 

• Directionality may also be determined when edge scallops appear on only one side of a bloodstain.

• Correct interpretation of bloodstain patterns must include consideration of the surface texture of the material upon which the bloodstains have been deposited.

• Surface tension prevents spattering regardless of the distance a drop of blood has fallen before impacting a smooth, hard surface such as glass.

• Edge characteristics of a bloodstain have no value in establishing the distance a drop of blood may have fallen prior to impacting a surface unless the nature of the surface texture is considered.

• Conclusions as to the significance of a limited number of bloodstains should only be drawn with reservations and/or qualifications. 

• It should be remembered that a few bloodstains do not a make a pattern. 

• There is nothing wrong with admitting that there are insufficient bloodstains available to form an opinion.   It is far better not to have an opinion than to have one that is incorrect.

• When a dozen or more small bloodstains are present in a recognizable pattern, their size may allow a prediction as to the energy that was required to produce them. 

• When the majority of individual bloodstain diameters are less than 1mm, they are consistent with having been produced as a result of a high velocity impact.  Most often they would result from a shooting.

• When the majority of dozens of individual bloodstains are approximately 1mm or more in diameter, they are consistent with having been produced as a result of a medium velocity impact.  Most often they would result from a beating or stabbing.

• The shape of a bloodstain is a function of the angle at which it impacts a surface. 

• Perfectly round bloodstains result from a ninety degree impact. 

• The angle of impact of an elliptical bloodstain may calculated from its length  to width ratio.

• Bloodstains may often be lifted from the surface upon which they have been deposited. 

• The harder and smoother the surface the more likely they may be successfully lifted.

• When measurements and angles are used to establish the origin or origins in space, not only will the actual origin be somewhere below the point or points of convergence.

• Sobriety of the victim will have no significant effect on how bloodstain patterns are produced. 

• A high blood alcohol level is of no concern to the interpretation of bloodstain patterns.

• When blood is projected upward with sufficient force to strike a ceiling, it will almost always be the result of a gunshot having an upward trajectory. 

• Such a trajectory is more often the result of a suicide rather than it is a homicide.