What is Blood Evidence?

Nicole Madison
Nicole Madison

In criminal investigations, blood used as criminal evidence may be referred to as blood evidence. For example, human blood found on a knife blade could simply mean a person has cut himself while chopping vegetables. It could, on the other hand, mean that the knife was used to stab someone. Blood evidence may be used not only in determining whether a crime has been committed, but also in identifying the victim and perpetrator of the crime.

Blood from a victim and perhaps a suspect might be found at a crime scene.
Blood from a victim and perhaps a suspect might be found at a crime scene.

In some cases, blood evidence may be used to determine whether a crime has been committed. For example, if a person is reported as missing, investigators may examine the home of a person suspected of harming him. The investigators may find drops of blood on the floor or other surfaces and collect and analyze them. Analysis of this blood evidence could reveal that the blood is human and not the suspect’s or that it is animal blood. If the blood is human, however, investigators may analyze it to determine whether it belongs to the missing person.

UV black light can be used to inspect for traces of blood.
UV black light can be used to inspect for traces of blood.

Sometimes blood evidence can also provide important information about the perpetrator of a crime. If, for example, there was a murder that involved a violent struggle, there may be lots of the victim’s blood at the crime scene. If the criminal who committed the murder, however, was also injured during the struggle, investigators may find his blood at the crime scene as well. This blood evidence may help investigators determine who the murder suspect is and may even help prosecutors prove their case in court.

In many cases, blood evidence patterns may help investigators solve crimes. For example, an investigator may examine blood splatters and drips to figure out the type of weapon that may have been used and how the crime was committed. In fact, a crime investigator may even use bloody footprints and fingerprints to help him retrace a criminal's steps.

In some cases, a criminal may attempt to hide his crime by wiping away blood that could be used as evidence. Cleaning up after committing a crime may not be a foolproof method of escaping justice, however. Investigators may use black lights and chemicals to find blood evidence a criminal has tried to erase. For example, investigators may use a substance called luminol and black lights to find traces of blood. If a trace of blood is present on the surfaces in question, it will typically glow when black lights are used.

A crime lab analyst may inspect blood evidence from a crime scene.
A crime lab analyst may inspect blood evidence from a crime scene.
Nicole Madison
Nicole Madison

Nicole’s thirst for knowledge inspired her to become a wiseGEEK writer, and she focuses primarily on topics such as homeschooling, parenting, health, science, and business. When not writing or spending time with her four children, Nicole enjoys reading, camping, and going to the beach.

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Discussion Comments


@MrMoody - I don’t know how often forensic DNA is the magic bullet in nailing suspects. However I do know that as a source of evidence blood DNA is almost incontrovertible.

I remember reading that at the DNA level, we were something like 99.9% similar, but it’s that small variation of difference that is enough to separate one person from another.

I would agree, therefore, that properly handling forensic crime scene evidence would be of utmost importance.


@David09 - I do believe that the bar has been set high to obtain criminal prosecutions. Whether it’s a result of the “CSI effect” I think is open for debate.

I believe that most jurors understand that the courtroom is not television, be it CSI or anything else. Blood evidence collection – or any forensic evidence for that matter – is a delicate procedure.

If it’s not done correctly, the defense will harp on that. In fact, from my experience in working on juries, it’s not uncommon for the defense to claim that the evidence was improperly handled, potentially tainting the evidence and making it inadmissible in court.

In my opinion, these are the real things which derail the cases brought on by the prosecution, not the CSI effect.


Here’s a disheartening fact that I learned when I served on jury duty. Forensic examinations are not always as conclusive as you see on television.

Blood evidence would certainly be bullet proof in most cases, but you don’t always get that. In the jury trial that I served on, there was very little forensic DNA, and it came back as inconclusive.

The prosecutor pointed out that this was typical for most forensic DNA samples, as if she wanted us to understand that DNA was not a magic bullet like the CSI evidence you see on television.

Basically what we had was a murder case based on witness testimony and circumstantial evidence. On that basis, the jurors could not find it in their hearts to send the suspect to prison.

They say jurors these days suffer from the “CSI effect,” where they expect things to happen the same way they do on that show. I don’t know if that’s true, but we certainly set the bar high.

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