Question: How much information can you approxematly get from a simple blood stain?

Mark Hill answered on 18 Mar 2011:

Hello Khadijahb,

Thanks for this question. A really good and important one in forensic science. I hope that I answer your question well enough.

The answer is simply – Lots.

There are two aspects to a blood stain – its composition and its shape/location.

First of all – composition.

Blood is made up from white cells, red cells and plasma. However, analysis of the condition of those components and their proportion can tell a lot about the health of the donor. The DNA can also tell a lot more, as you can imagine, but red cells don’t have a nucleus, so only mitochondrial DNA is available, which is inherited from the female line – the mother. A full profile DNA set may be obtained from white cells.

Within the blood stain drug and alcohol testing (toxicology) can reveal lots, about drunkenness of the individual, doping, or prescribed medicine for an illness or medical condition.

Oxygenated blood is bright red, Deoxygenated blood is dark red. So a fresh blood stain, or spatter, can tell us whether it was an arterial or a venal bleed and wound.

Now for the shape and location.

Arterial wounds tend to spurt, due to the direct pressure of the heartbeat, so spatter in jets gives a different pattern to venal blood, which tends to be consistent. The amount of blood also suggests the severity of a wound and the movements of the victim. Blood droplets cam tell us a lot.

Here’s a little but interesting experiment for you and your science teacher to try.Drop a droplet of a thickish liquid, say full cream milk, onto a smooth surface. If the surface is flat, then the droplet will be broadly round. That gives the angle of delivery at about 90 degrees. Now tilt your surface slightly and repeat. The droplet pattern is conical. The thinner end is furthest from the point the droplet landed. Thus, shape of droplet cam help give us the direction of delivery, how the person was standing and where.

There are certain maths formulae, all simple trigonometry, that allow us to be more precise. So, with the above, we can tell a lot about the donor, how the moved and where they moved. The study of blood patterns is spookily enough known as Blood Pattern Analysis and is a detailed science field in its own right.

A brilliant question and one that you can see I am really interested in. I could have written loads on this as I use it occasionally to work out how un-belted car occupants fell around in a car in a collision, although their injuries also help in working out the ‘kinematics’ of where they were likely to have been sitting and who was probably the driver.

Thanks,

Mark.