April 08, 2003: More information regarding to Serology has been added. Click here.

Blood is perhaps the most valuable evidence in the world of forensic science because its presence always links the suspect and victim to one another and the crime scene.  Bloodstain or spatter patterns explain a lot about positions and movements during the crime.  It can provide information such as who struck whom first, in what manner, and how many times. Scrupulous analysis can destroy alibi and self-defense arguments for a crime.  

Blood is red fluid that is composed of red cells, white cells, platelets, and plasma.  Each person’s blood has certain inherited characteristics that distinguish it from other people’s blood.  Blood type is detected by simple antibody-antigen interaction.  Antibodies, which are found within plasma, distinguish particular antigens from others.  Either bursting or agglutination of the red cells takes place when foreign antigens are found.  In the case of agglutination, the antibodies bind the antigens from different red cells.  As you can see in the figure on the left-hand side, it shows what you have to look for in ABO blood test result.  Agglutinated blood or clumps indicate the presence of various blood types as shown in the table below.

For more information about blood typing, please click here.

Prior to discussing blood spatter analysis, there are important laws of physics concerning liquids that students need to understand.  Liquid particles exert electromagnetic force of attraction on each other.  These forces are called cohesive forces, which affect the properties of liquid extensively.  On the surface of liquids, cohesive forces become stronger than particles in the inner layers of the liquid because there are no neighboring atoms above.  Therefore, it exhibits stronger attractive forces upon their nearest neighbors on the surface.  This is called surface tension which is the tendency of the surface of a liquid to contract to possess smallest area possible.  So how does surface tension play a role in the blood spatter analysis?  First, gravitational force acting on blood must exceed its surface tension before a drop of blood can fall.  Otherwise, there will be no blood drop since they will behave like slime or sludge.  Second, no matter how high a drop of blood falls, surface tension minimizes spattering before impacting a smooth, hard surface such as glass.  In other words, a drop of blood will scatter away into many small spatters without surface tension.  It would put forth more problems to the investigators, who would be wasting tremendous time by collecting pointless evidence.

Crime scene investigators often want to reconstruct the exact position where the crime has occurred.  This is where a blood spatter analysis is brought to attention.  A blood spatter analysis can provide two pieces of information.  One is the direction in which the droplet was traveling, and the other one is the impact angle.  Impact angle is the angles at which blood droplet struck a horizontal surface such as the floor.

Shapes of blood spatter are either round or ellipse to oval shape. If blood drops from directly above, it will have round shape depending on the surface texture. If there is some impact on the droplet, such as from a gunshot or any penetrating wounding agent, an elliptical or oval shape is formed.  Long axis of the ellipse indicates the direction of blood that has been traveled from.   From this analysis, the point of convergence in two-dimensional plan can be determined. 

Thicker side of the elliptical stain is where blood has traveled.  Thus, opposite of the thicker side (thinner side with tail looks) is where the origin of blood droplet was coming from.  Generally, the impact angle of blood on a flat surface can be obtained by measuring the degree of circular distortion of the stain.  By using the equation shown above in the diagram, impact angle is calculated with width and length of the bloodstain.  This measurement, combined with the point of convergence will locate the origin in three-dimensional space.   Because there will be a lot of blood spatter in a crime scene, investigator usually draws lines with strings at the crime scene or use CAD software to improve the view of blood flight paths.  Such programs can be found at Blood Spatter Analysis with Computer. 

In addition, Herbert Leon MacDonell is the leading authority on blood stain interpretation. His remarkable observations have led the way in this particular field. In his published study, Flight Characteristics and Stain Patterns of Human Blood, he gives the following tips to the investigators:

·        As shown on right-hand side, surface texture is one of the key components in determining spatter type. By this, MacDonell means that the harder the surface, less splatter will result. It is therefore extremely important to duplicate the surface in a controlled test.

·        When a droplet of blood hits a surface that is hard as well as smooth, the blood usually breaks apart upon the impact. This in turn causes smaller droplets. The smaller droplets will continue to move in the same direction as the original droplet.

Forensic biologist, Thomas M.Kotowski has provided some of his collection.  As you can see from the top picture, speed has influenced blood spatter significantly. Elliptical shape of bloodstains becomes longer and sharper as speed increases.  Another picture shows that the height of blood drop was 24 inches. See the difference with previous one?  Last pictures indicate various bloodstains by impact angle. Perfectly round bloodstain results from a ninety-degree impact, while longest sharp bloodstain results from ten- degree impact. 

Becoming an expert on blood spatter interpretation is something that cannot be achieved in a short period of time. It requires a lot of reading, field experience and practice. For general rules of thumb, one may visit Blood Spatter Interpretation website.