Understanding Bloodstain Patterns Better

Forensic evidence isn’t present, let alone conclusive, as often as you might see on TV. 

Shows like CSI and Bones, for example, could easily lead viewers to expect exact DNA matches every time. But in reality, exact DNA matches technically don’t exist — and forensic evidence isn’t the deciding factor in criminal cases as frequently as popular shows and movies portray.

Because blood’s easy to show on-screen, the police officers and prosecutors in procedural crime dramas regularly rely on it to convict their bad guy. In real life, though, it’s not so easy to prove a case based on bloodstain patterns alone. 

It can be difficult to accurately assess things like how quickly blood was traveling…

Historically, bloodstains have been considered reliable evidence. Bloodstain pattern analysis has been admissible in court for over 150 years; in 2009, however, its validity began to come into question. 

In a 2016 study, 75 practicing bloodstain pattern analysts were asked to examine 150 distinct bloodstain patterns using 192 total images. As it turned out, these highly trained forensic analysts came to conclusions that did not match the known cause of bloodstains they were asked to examine more than 10% of the time. The same study found the rate at which any two analysts’ conclusions contradicted each other to be roughly 8%.

Several factors can contribute to differing interpretations — and dubious conclusions — drawn from bloodstains. The surface material, for instance, makes a big difference. Sometimes it can be hard to even say for certain how exactly the blood came in contact with the surface. 

“It can be difficult to accurately assess things like how quickly blood was traveling when it struck the fabric,” says Tiefang Wang, a professor of mechanical and aerospace engineering at NC State University. “Was it going fast? Slow? Did someone just brush up against the blood? It’s hard to tell, because once the blood comes into contact with the fabric, it wicks across the surface of the fibers in the fabric, spreading out.”

But Wang and his colleague’s recent research could now make it easier for forensic analysts to determine factors like velocity, at least when working with things made of cotton. Detailed in a paper published in Forensic Science International, on which Fang was a corresponding author, the research uncovered new findings on how bloodstains form on cotton fabrics — which “can be used to provide insights for forensic investigations.”

“We’ve now done analysis that advances our understanding of how these bloodstains form and can be used to estimate the velocity blood was traveling when it struck cotton fabrics,” Fang says. 

The researchers splashed five different fabric surfaces — plain-woven cotton fabric, which has the same surface characteristics on the front and back, as well as the front and back of cotton twill and a jersey knit, both of which have different characteristics — with blood at 12 different velocities. They used a set of high-speed cameras to capture the impact of the blood at a rate of four frames per millisecond.

A key finding, according to Fang, related to the “fingers” of a bloodstain.

“When you look at a bloodstain on fabric, you will sometimes see thin tendrils that spread out from the center of the stain. Those tendrils are referred to as fingers,” Fang explains. “We found that the more fingers a bloodstain has, the faster the blood was moving when it struck the fabric. However, over time, these fingers may spread out and run together.”

Fang and his fellow researchers’ findings could one day help courts ensure the right person is put behind bars for a violent crime. But the team also acknowledges that there’s much further research to do on how blood interacts with different types of fabric. 

“The physical properties of plain-woven cotton make it much easier to estimate the velocity of blood spatter compared to the other fabrics,” Fang says.

Twill fabrics, on the other hand, make it the most difficult to estimate the velocity at which the blood spatter traveled.

“The results of this study are promising, and we’re interested in doing additional work with a wider variety of fabrics, weaves and yarns,” Fang says. “It’s clear that the specific structures of each surface play a critical role in how these bloodstains form and what we can learn from them.”

This article is based on a news release from NC State University.