Blood and other bodily fluids which are found at the crime scene may give important physical evidence in three ways;
1)The occurrence of a blood strain in a certain place, e.g. on a weapon may substantiate an account of a crime;
2)The shape, position, size or intensity of a bloodstain may support a particular sequence of events; and/or
3)The blood typing analysis can be used to eliminate whole groups of people as suspects.
It is therefore important to be able to identify a particular stain as blood or not, or maybe even to reveal "hidden" bloodstains on dark materials or where attempts have been made to wash the blood away.
Human blood contains a pigment called haemoglobin, which is used to transport oxygen around the body. This pigment is used by a number of tests to identify the presence of blood. One particular test that reveals the presence of blood is the Luminol Test. In this test the bloodstains can be made to glow with a blue light due to the chemiluminescent reaction of the luminol reagent with the iron in the haemoglobin.This test is sensitive enough to pick up minute traces of blood even when attempts have been made to wash away the incriminating evidence.