Forensic Blood Analysis - How Do CSI's Know If They Are Dealing With Blood?

Whenever a bloodlike stain is found at the scene of a crime, the forensic scientist must conduct two basic kinds of tests: presumptive and confirmatory. The reason presumptive tests are conducted first is because they are more cost-efficient than the more cumbersome confirmatory tests.

What is a presumptive test?

Presumptive tests for blood can be found in two broad categories: those that change color, and those that cause a glowing reaction.

Presumptive tests that depend on a color change involve the following:

• Leucomalachite green (LMG) color test: This chemical reagent has been around since the early part of the 20th century and undergoes a chemical interaction with blood, yielding a characteristic green color.


• Tetramethylbenzidine (TMB) color test: At a crime scene, a CSI technician swabs a suspected bloodstain with a moistened Q-tip and then applies it to a Hemastix strip containing TMB. A Hemastix strip is a dip stick used to test for the presence of blood. If the Hemastix strip turns blue-green, it might be blood.


• Kastle-Meyer color test: Phenolphthalein is the active chemical reagent in this particular test. When blood, hydrogen peroxide, and phenolphthalein are mixed together, a dark pink color results. This color change is due to the hemoglobin (the oxygen-containing molecule within red blood cells) causing a chemical reaction between hydrogen peroxide and phenolphthalein.

Other tests depend on chemical reactions that cause blood to fluoresce, or glow, under an ultraviolet light source, uncovering blood that is invisible to the naked eye. Offenders many times try to scrub walls and floors clean, mistakenly taking for granted that if blood is not visible, it can not be found. Fortunately, that is not the case. At the scene of a crime, blood can show up by spraying a fluorescent chemical over the area suspected of containing blood. The lights are dimmed, and ultraviolet light is passed over the area, causing bloodstains to fluoresce in the dark just like you see on CSI. Not only is the presence of blood established, but the area of blood distribution is also clearly indicated. This is where blood spurts, drag marks, spatter, footprints, and handprints show up.

The actors you see on CSI use fluorescent chemicals to get the blood to reveal itself. The most common fluorescent chemicals used are luminol and fluorescein.

• Luminol--Luminol is highly sensitive and may reveal blood that is present in minute quantities. Luminol is able to expose blood in places that have been cleaned repeatedly unless a solvent such as bleach was used, and even on walls that have been painted to hide blood. In spite of the fact that it can negatively impact some serologic testing processes, luminol does not affect subsequent blood typing or DNA analysis.


• Fluorescein--Fluorescein has been around since the early part of the 20th century. This compound does not interact with bleach the way luminol does. Fluorescein is better used over luminol for exposing bloodstains that have been cleaned up using solvents such as bleach. Fluorescein is advantageous in that it is thicker than luminol and drips less, therefore, sticks to vertical surfaces much better than luminol.

What is a confirmatory test?

Teichmann and Takayama tests are the most commonly used confirmatory tests. Both tests depend on a chemical reaction between a reagent and hemoglobin. This reaction yields crystals, which then can be seen under a microscope. A considerable benefit of these tests is that they are more effective with aged stains.

The next time you watch your favorite forensic science/detective show like NCIS or CSI, you will have a clearer understanding of the chemicals used in exposing blood at a crime scene.

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