Now that you have some basic knowledge of DNA and the processing used by forensic scientists, let’s look at the difference between presumptive tests and confirmatory tests.

When a possible blood stain is found at a Crime Scene a presumptive test is performed. There are many types of presumptive tests and you have probably seen some of them if you watch any of the CSI shows.  Presumptive tests tell investigators if the stain could be blood.  There are substances that can give false positives also so the blood must be tested using a confirmatory test.  A confirmatory test confirms that the stain is for sure blood.  After a stain has been confirmed as blood, forensic scientist must then determine if it is animal or human blood and who the blood belongs to if it is determined to be human.   

Let’s take a closer look at some presumptive and confirmatory tests. Go the following websites to learn about two presumptive tests: Luminol test and Kastle-Meyer color test.

• Chemistry World - Luminol
• Luminol: A Glow-in-the-Dark Reaction
• Kastle-Meyer color test - You may read the section on this test and skip all other materials.

The Takayama test will produce crystals if the sample is truly blood and then from there scientists can determine whether it is human or animal blood.  If it is human blood the forensic scientist completes the DNA analysis and compares it to known DNA samples.

• Forensic Resources - Takayama Test - Scroll down to the section on blood and read about the Takayama test only.