Ellen Phiddian
Cosmos science journalist
Bloodstain pattern analysis: you may have seen it on the TV, or heard about it on a true crime podcast.
The idea of scientists seeing into the past and figuring out how a fight happened, based purely on a few drops of blood, is intriguing.
But how does it go down in reality?
To learn more, Cosmos sat down with Professor Adrian Linacre, Chair in Forensic DNA Technology at Flinders University. Linacre has spent decades looking at crime scenes, including 17 years visiting crime scenes in Glasgow.
“I went to some pretty horrendous murder scenes,” he says.
“When I talk about blood pattern, I wax lyrical about it – but you have to realise that somebody suffered.”
Blood pattern is also not an exact science. But it is still a useful tool for unpicking a crime scene: so, how does it work?
What is bloodstain pattern analysis?
“It is a reconstruction tool,” says Linacre.
“It’s based upon looking at the patterns of blood at crime scenes or on items, and thinking: how best can I think about how that got there?”
This means thinking about the size, shape, and distribution of bloodstains at a scene.
“The crucial word for us is patterns,” says Linacre. “One bloodspot doesn’t help. We look at patterns.”
They rely on the science of blood to do this: its biology, chemistry, and especially physics and maths. It relies a lot on fluid dynamics, and Newton’s laws of motion.
Both the size, and the shape, of blood droplets can give you an indication of how they got there.
“There’s a marvellous physical relationship between the size of the blood spot and the force applied: the greater the force, the smaller the blood spots,” says Linacre.
Blood dripping passively from a wound, without any other force acting on it, will form fairly big drops: around 50 microlitres, or 0.05 mL.
“That’s, energetically, looking at surface tension, cohesion and adhesion, how blood wants to be,” says Linacre.
When they hit the ground, these drops will form stains a few millimetres in diameter.
“As soon as you see tiny blood spots, i.e. little pinpricks under one millimetre, that’s high force. That cannot happen by someone just dripping blood,” says Linacre.
This means that small spots have been made by some sort of force – like a kick, a punch, or a stabbing or shooting.
Tiny drops also can’t travel very far before wind resistance slows them down: so small drops at a crime scene are usually very close to the event that caused them.
“You can look at the distance of blood spots, and work out what force was applied, and how far it most likely went,” says Linacre.
Want to learn more about this? Listen to our podcast on bloodstain pattern analysis, The Science Briefing.
The shape of the bloodstain can tell you what direction the blood was travelling in.
“When blood lands on a surface directly downwards, it makes a round stain,” says Linacre.
“If it lands at an angle other than 90°, it makes an elliptical stain. Therefore, you’ve got a width and a length of that stain.”
You can measure the width and length of a bloodstain, and plug it into an equation, to figure out the angle the blood was travelling at when it hit the surface:
Angle of impact = sin-1(width/length)
“It’s a simple equation, but it works,” says Linacre.
While there are a few elements of the blood droplet that can be a little more complicated – bigger drops sometimes create smaller additional drops on impact, for instance – you can still figure out the angle the blood was travelling at with high school trigonometry.
“When you look at stains on a wall or surfaces, you can mathematically calculate the angle – the brain is also pretty good at it – and calculate, therefore, where did they all come from,” says Linacre.
Forensic scientists can map out the “points of origin” in an area: the places where bloodshed happened.
“Although you weren’t there, you don’t know for definite, you can come up with some pretty good scenarios as to what is the most likely way in which this happened,” says Linacre.
Researchers also experiment with blood movement and fluid dynamic modelling in laboratories to figure out how blood might behave in more complex scenarios – for example how gases from a gunshot move them around.
How reliable is bloodstain pattern analysis?
Linacre says that bloodstain pattern analysis is used more in courtrooms now, than when he started giving evidence in 1994. The reason? DNA analysis.
“In years gone by, when I started, often the argument in court was ‘whose blood is that?’ That’s gone. Defence accept it, prosecution accept it, move on.”
Knowing whose blood it is makes pattern analysis much more powerful. It also means that “how did that blood get there” has received more focus in courtrooms.
But forensic scientists never use bloodstain pattern analysis to make accusations.
“Say we’ve got lots of tiny spots on the front of a shoe, and that DNA came from someone who sustained, looking at the medical report, many injuries to them. That shoe must have been in very close proximity to a source of wet blood when it sustained lots of impacts,” says Linacre.
“We would never ever, ever say in court, ‘therefore, this person kicked that person’. That’s never our job. The jury do that. We aid them by saying, ‘in our opinion, that shoe was in very close proximity to a source of wet blood when it sustained forceful impacts’.”
Linacre also notes that bloodstain pattern analysis is never going to be as definitive as other forensic evidence, like DNA.
“Blood pattern is ultimately a subjective judgment. It is based upon the experience and knowledge of the examiner.
“We try and remove that a lot from forensic science. We don’t like subjective judgments, we’d rather as much empirical data as possible. Blood pattern can fall under that problem of being, ultimately, an opinion.”
But it’s still based on very robust physics.
“It’s very, very difficult to account for it in any other way,” says Linacre.
He does wonder, though, if bloodstain pattern analysis is seen as more convincing than other, more reliable, pieces of evidence.
“With DNA, I would trot out, ‘it is 100 billion times more likely that the blood came from Adrian Linacre compared to anyone else from your local population, taken at random by chance’. It’s a mouthful.
“Whereas with blood pattern, you say ‘in my opinion, that blood came from about a metre away, high force, typical of actions of kicking into a source of wet blood’. I think the jury can picture that easier.”
Bloodstain pattern analysis is a useful tool, but it’s never going to give you certainty.
“We weren’t there. We didn’t see happen,” says Linacre.