Blood, Presumptive Test

A forensic investigator can be confronted with a variety of fluids at a crime or accident scene. It is critical to determine the nature of each fluid.

While a detailed examination of a suspect bloodstain requires the equipment and technical expertise of an analysis laboratory, a fluid suspected of being blood can be examined at the scene to determine if it indeed could be blood. This examination is called a blood presumptive test.

Properly done, a blood presumptive test rules out the possibility that a fluid is blood. A blood presumptive test relies on the use of chemicals that will change color when in the presence of blood. As one common example, a solution of phenolphthalein, which is colorless, will turn an intense pink when added to a blood stain in the presence of hydrogen peroxide. The formation of a pink color indicates that the fluid could be, and indeed, likely may be, blood. However, confirmation requires the more detailed lab analyses.

Another chemical called o-tolidine can also be used in conjunction with hydrogen peroxide instead of phenolphthalein. Again, the formation of the characteristic color must be followed up by a more detailed and confirmatory examination of the sample.

When a blood presumptive test is done at a crime or accident scene, an investigator must include the use of controls to ensure the accuracy of the result. This is because a blood presumptive test can be subject to what is known as a false positive result. This is when the characteristic color reaction is produced by a sample that is in fact not blood. As well, a false negative reaction is possible, where for some reason a blood sample does not produce the characteristic color change in the indicator chemical.

Standard procedures can rule out the possibility of a false positive or negative result. However, if these controls are not run, then the accuracy of the presumptive test can be questioned. In that case, the results would not be admissible in a court of law.

Presumptive blood tests are commercially available in a convenient form that is easily transportable to the crime or accident scene. Typically, a sample is placed in a sterile plastic bag or box to which are added the chemicals. Upon mixing, the solution is visually observed for the development of the target color. Other containers contain the positive and negative controls.

see also Blood; Blood spatter; Bloodstain evidence; Blood volume test; Indicator, acid-base.