How Does Luminol Detect Blood at Crime Scenes?

Do you want to use luminol at the crime scene in your fiction book? This post tells you how luminol works, how the luminol test is performed, what types of crime scenes and evidence are ideal for using luminol, and some notes for using it in your crime fiction novel.

You’ve probably seen luminol used on crime shows on TV, and you’re thinking about using it in your crime fiction novel. Luminol is a great tool to use at crime scenes and in crime labs to look for blood.

If you use luminol in your book, you want to make sure it’s accurate.

In this post, I explain what luminol is, how luminol works, and a short list of things you should know when using luminol in your crime fiction novel.

How Does Luminol Work?

Luminol is a chemical that glows blue (similar to the photo above) when it is mixed with an oxidizing/alkali agent. The addition of heme, which is found in red blood cells, enhances this reaction.

Luminol is a presumptive test for blood. Presumptive tests are quick tests used to screen evidence for the presence of body fluids. If a presumptive test is positive, it indicates the body fluid is likely present, but the test does not officially identify it.

Other chemicals can cause a positive reaction with luminol (see below), so confirmatory tests are needed. Substances known to cause a positive reaction with luminol are bleach and other cleaners, certain metals, and plant material.

Luminol is not specific to human blood. Animal blood will also show a positive reaction.

Luminol Test Procedure

The process described below gives a general idea of how the luminol test is performed. Official protocols vary by crime lab/agency.

The test consists of two separate reagents. Just before use, the two reagents are combined into a working solution in a plastic spray bottle for activation. The standard operating procedure (SOP) in my old crime lab required us to wait 5 minutes before using it.

The area/room where the luminol test is being performed must be completely dark. I once saw an episode of CSI Miami where they used luminol in broad daylight and the blue glow was visible, which definitely does not happen.

Luminol is applied by spraying the activated solution over the area/item using a fine-mist spray bottle. If blood is present, you should see a light blue glow right away. The reaction usually lasts about an hour.

A minimum amount of reagent should be used so the stains don’t run and mess up the pattern if they’re on vertical, nonporous surfaces (walls, etc.). Minimal use of luminol is also important so as not to dilute any blood that is present for future DNA analysis.

If there’s a positive reaction, the area is marked and/or photographed.

In the crime lab, we used luminol to help us figure out where the blood was, and then we swabbed the area or took a small cutting of the evidence for DNA testing. We documented positive areas on a diagram in our notes. We did not take photographs of the luminol reaction. (This could be different at other labs.)

At crime scenes, the luminol reaction is normally photographed by the CSI first to document the pattern, and then the item is collected and packaged.

When Is Luminol Used at Crime Scenes?

The first thing a forensic scientist or CSI does when examining an item or scene for the presence of body fluids is a visual inspection for stains. If a red stain is visible, then the item is collected and luminol is not needed.

Luminol is used at crime scenes and in crime labs on dark/patterned items or locations where blood is not easily visible. Think an ugly couch from the 1970s, patterned wallpaper, and dark fabrics.

Luminol can detect very dilute amounts of blood.

The CSI can also use luminol if they believe the perpetrator tried to clean up the crime scene. They never get everything and it’s possible some blood was sprayed into the floorboards, under cupboards, and in other sneaky places.

Luminol in Action

I only recall using luminol two times in the 7 years I worked at my old crime lab.

The first time I was in training. I was observing a co-worker examine items of evidence from a double homicide. She was screening a black t-shirt and kept getting a very slight reaction using leucomalachite green (another presumptive test for blood), and she couldn’t narrow down what part of the shirt it was coming from.

When she performed the luminol test, we both saw a light blue glow on a few small areas on the shirt. She marked them with glow-in-the-dark tape and later took swabs of the areas on for DNA testing.

The second time was when I had to officially perform the luminol test as part of my training. A trained forensic scientist had to watch me prepare the reagents, perform the test, and properly record the results. I didn’t actually use luminol on real evidence.

Additional Notes for Using Luminol at the Crime Scene in Your Book

Older blood reacts more strongly than fresh blood. Luminol is particularly useful at old crime scenes, where the blood may be harder to see. It’s not needed as much at new crime scenes where the blood is fresh (although it can still be used).

Distilled water should be used to make luminol. Tap (“hard”) water should not be used to make luminol because the substances in tap water interfere with the reaction.

A company called BlueStar Forensic produces a commercial form of luminol. There are three tablets and a liquid reagent. The three tablets are added to the liquid reagent to make the activated working solution, which is then sprayed on the item/area.

Remember that the room/area must be completely dark when performing the luminol test. The potential blood on the item will glow blue. The CSI/forensic scientist is not using an alternate light source (blue flashlight) to find the luminol. I see this myth a lot on TV (and in stock photos).

Summary

You now know what luminol is, how it works, and a few things to keep in mind when using luminol in your crime fiction novels. Be sure to follow the guidance in the post to help make luminol more believable in your crime fiction novels.

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Photo credit: Adam Kozak / CC BY-SA