Homicide detectives know too well the value of blood stain analysis. The pattern of a stain can confirm an alibi or be a catalyst for a confession.

It can put offenders in a particular place at a particular time. It can identify a victim. It can reveal the presence of one or more people. It can point to a murder weapon.

Sergeant Melissa Bell knows too well the truth in Shakespeare's prose, "Blood will have blood". Blood sticks. It does not lie. A tiny spot may be all that's needed to solve a crime.

On May 22, 2009, Sergeant Bell was called to a Goomboorian farmhouse just outside Gympie.

There, the body of Graeme Hughes was slumped in a bathtub.

His head had been caved in so severely that the back of his skull was a spiderweb of fractures.

The bathroom was a bloody mess.

"There was significant blood-letting within the bath and in the area around the bath," Sergeant Bell said.

"There was a hammer located in a kitchen sink and it was soaking in water and what we believe to be bleach in that water."

Wearing a blue forensic suit, known among those in the Scientific Section of the Major Crime Unit as "Smurf suits", Sergeant Bell sets about examining the scene.

Chiefly, she takes note of the wounds on the victim, the position of the victim and the blood stains on the victim.

She then notes the size, shape, distribution and location of the bloodstain patterns.

Sergeant Bell identifies bloodstain patterns within four categories: passive, projected, transfer or altered bloodstains.

In the bathroom of the farmhouse, Sergeant Bell identified multiple projected bloodstains inside the bath and on the walls and floor.

That indicated the victim had suffered more than one blow to the head.

Sergeant Bell also identified projected "cast-off" bloodstains on the walls surrounding the bath and on the vanity and the floor.

She determined the cast-off blood spatter on the wall resulted from a bloodied objected moving in a downward direction.

The absence of blood on the outside of the bath – a "blood void" – indicated the victim was in the bath when he died.

"There was [an area void of blood] outside the edge of the bathtub, one to two tiles in width, which may be explained by blood being projected downwards and shielded by the lip [of the bathtub]," Sergeant Bell would later tell the Supreme Court.

"The void was quite large so [the victim] was not very high above the bath [when he was struck]. It's likely the impact happened within the bath to create that void area. It was less likely he was standing in the bath."

The findings were crucial to the prosecution case against Mr Hughes's killer Allan James Murray, who claimed he was acting in self-defence when he struck his employee six to seven times. Murray claimed Mr Hughes had first swung the hammer at him.

The blood drip trail from the bathroom to the kitchen sink matched Mr Hughes's DNA.

Back at the laboratory, Sergeant Bell found traces of blood on the head of the hammer.

Suspected bloodstains are first screened with the chemical tetramethylbenzidine, commonly referred to as TMB. The TMB test is a presumptive test for blood.

The positive presumptive test was supported by subsequent DNA tests.

Sergeant Bell then recreated the crime scene at the lab, striking mannequins from different angles and in different directions with the hammer, which was retrieved from the house.

The experiments, although not accepted as evidence in court, supported her initial findings.

"I got very similar bloodstain patterns as to those that were at the scene," she said.

The victim had been bludgeoned to death with the hammer while he was in the bath. He was not standing at the time. He was not outside the tub.

The Supreme Court jury accepted Sergeant Bell's initial evidence, finding Murray guilty of manslaughter. Murray was sentenced to nine years' jail in 2011.

Crime scenes are not always so easy to examine and reconstruct.

"I've had a job where I've gone and looked at blood and it has been difficult for me to then categorise that blood and so I'm unable to use it in a reconstruction," Sergeant Bell said.

"I had another job that was a person who was shot with a rifle at a range that it could have been both a homicide and a suicide. At that close range you would have the same bloodstain patterns if it was a homicide or a suicide. That's one of the limitations of bloodstain analysis."

In those cases, firearm and fingerprint specialists are called in.

Sergeant Bell approaches crime scenes with clinical objectivity, but views her role as one that gives a voice to victims.

"I'm working for a victim who can't speak for themselves," she said.

She values the opportunity to use her skills and scientific knowledge in real-world practice.

"It's kind of CSI stuff, but only first-season CSI," she said.

Sergeant Bell spoke about bloodstain pattern analysis as part of a series of public lectures held on the last Sunday of each month at the Queensland Police Museum.