Blood and Stain Analysis

Blood can tell us a lot of things. It is rich in information. For example:

• It contains DNA which can be extracted for genetic fingerprinting. Or perhaps the owner suffers from a disorder such as sickle-cell anemia (due to the Malaria parasite)
• The blood type can be determined. While not sufficient to solve a crime, blood typing can narrow the search and lead to verification of other data
• Blood may be attacked by bacteria or insects, helping to reconstruct a time line of events
• The shape of spattered blood stains can be used to reconstruct the position of the blood source (the injured person)
• It leaves a visual trail that can be followed for some distance

Apart from the blood-types we are familiar with (A, B, AB ,O and Rh factors etc), that can be quickly determined from chemical kits, the form, location and pattern of the stain are also very informative.

Go to the Blood Typing Tutorial from the University of Arizona Biology Project

Blood-stain enhancement

Luminol is a fluorescent chemical that can expose trace amounts of blood. More recently, chemical reagents that do not require special light sources, such as Hema-Glow ™ have been invented. However both these reagents degrade the cell DNA and are not recommended for cases where DNA is required and the amount of available blood is small. 

Forensic bloodstain experts also use sophisticated computer enhancement techniques to gain information that may be difficult to access from crime scene photographs.

images courtesy of Norman Reeves

Blood Spatter Analysis

When drops of blood fly everywhere they obey normal laws of physics. They move through parabolic arcs before striking a solid object. Since blood is quite viscous, it forms clearly shaped drops on surfaces it reaches. A drop falling vertically on a flat surface will produce a circular shape whereas a drop striking a surface at an angle will produce an ellipse. Forensic technicians can use the shape and the axis of the ellipse to calculate the trajectories (paths) of individual droplets.

Look at a more detailed analysis of blood spatter