Blood Typing Lab

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Forensics Mystery Blood Typing Lab

Scenario: Police are called to a house where they discover a body near the back door. There is a fresh pool of blood by the victim’s head. Officers question the neighbors and learn that a plumber and an electrician made service calls to the house just hours before. They were able to trace both. They recover a shirt with a dark stains from the service van of the plumber and rags with similar dark stains from electrician’s van.

Your forensics lab receives the following items to test:

Sample 1: Stained cloth from plumber’s van

Sample 2: Stained cloth from the electrician’s van

Sample 3: Blood collected from under the victim’s head

Test Background: The phenolphthalein test is a highly sensitive, being capable of detecting blood spots practically invisible to the eye. In an actual criminal case, a positive phenolphthalein test would be followed by tests to determine the species that produced the blood and if these tests indicate the blood is human, by tests to determine ABO, Rh and other blood groups. There is no point in pursuing typing and species identification procedures if the phenolphthalein test is negative. The phenolphthalein test is not absolutely specific for blood because most plants and some animal cells can also cause a positive reaction.

Discussion Questions:

1. Does a positive phenolphthalein test prove that a stain is caused by blood?

2. Does a positive phenolphthalein test prove that a stain is caused by human blood?
Procedure 1:

1. Test the shirt and rags to determine if the stains are caused by blood. 2. Place a drop of phenolphthalein/peroxide solution on each stain. 3. Observe if a color change occurs. A purple-violet color reaction indicates that blood (heme) is present (positive reaction). Absence of this reaction indicates that blood is not present. 4.