Updated 10 March 2014

Bloody clues help solve crimes

Blood spatters at a crime scene contain important evidence, if you know how to look for it.

What the untrained eye sees as gory messes in crime scene photos, are, to the blood stain (or blood spatter) analyst, meaningful patterns and potentially important pieces of evidence.

These are some of basics that a forensic investigator might "read" in different bloodstains:

Passive blood spatters

A large, round bloodstain is likely to have formed from a blood drop falling to the floor or other horizontal surface.

The larger the diameter of the stain, the greater the height from which the drop fell. This type of stain often occurs after a victim is wounded, not at the moment the wound is received.

For example, if blood drips from a stab wound on the victim's arm onto their sleeve and to the floor, the blood stain or spatter pattern formed is known as a low-velocity, "passive" spatter.

Blood transfers

Where a bloody object, like a hand or clothing, drags or smears blood across a non-bloody surface.

Medium- and high-velocity spatters

This is where the patterns start to get more complex and challenging to decipher. When a force ruptures the skin and impacts blood directly, it breaks up into droplets that land on surfaces in a specific spatter pattern.

Weapons produce different spatter patterns depending on the velocity at which they strike.

Medium-velocity blood stains are produced by striking with a blunt object such as a cricket bat or violent fist blow; stabbing can also produce this pattern. A medium-velocity droplet stain tends to have a maximum diameter of 4mm.

High-velocity spatters, as from a gunshot wound, are usually from a fine spray of droplets that form stains under 1mm in diameter.

Sometimes, however, a medium- or low-velocity spatter can look like a high-velocity spatter, because of cast-off blood droplets from larger drops.

The blood spatter can also indicate the number of shots or blows, and whether the attacker was right- or left-handed: right-handed assailants generally strike the left side of the victim's body, and vice versa.

Pinpointing the exact attack location

When blood leaves the body and moves through the air, it forms into liquid spheres. When one of these spheres lands on a surface, the spherical shape distorts and the resulting stain helps analysts determine the direction in which the blood was travelling.

A circular stain indicates the blood hit the surface pretty much straight on, at a 70-90 degree angle. As the angle of impact increases, the blood stain gets more oval and develops a "tail", which points in the direction the blood drop was travelling.

The flight paths of several droplets form vectors and when investigators trace these back and see where they converge, they show their area of origin i.e. where the blow or shot to the victim's body took place.

Timing the crime

Blood on a surface also dries in a characteristic manner, from the edges of the stain inwards. This can help determine when the crime occurred.

Also, on exposure to air, blood starts to clot. Blood spatters on a surface that show different degrees of clotting may indicate multiple blows or gunshots delivered over time, and when these occurred.

Brodbeck, S (2012). Introduction to blood stain analysis. SIAK-Journal. Journal for police science and practice.
Smithsonian Channel. Blood spatter 101.

Blood spatter image: Shutterstock

Olivia Rose-Innes is Health24’s EnviroHealth Editor. Read more of her columns and articles or post a question to her expert forum.


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