Forensic investigation techniques: where’s the blood?

Reports of lifeless bodies turning up are a sad recurring theme in South Africa’s newspapers: from the Oscar Pistorius case, to the more recent “axe murders” of the van Breda family, and the even more recent discovery of the body of a young woman on the mountains above Kalk Bay. An element of forensic investigation is present in all these cases, and busies itself with the answering of a few important questions: the who, and determining what happened – to name but two.

In the Oscar Pistorius case, for example, the “who” was evident given that Reeva’s body was discovered at the crime scene. That it was her blood on the floor was also obvious, thus negating the need for DNA tests. One of the key questions that had to be answered was how the blood got there – did it splash, drip, spray, etc.? The answers to these questions provide important clues with regard to the location of the victim and the perpetrator, and also a little insight into the events as they occurred.

Often, however, forensic investigators are faced with scenes where clues aren’t as obvious or plentiful. Imagine, for instance, the discovery of a lifeless male body on a tiled floor in an otherwise tidy kitchen. A single bullet hole in the chest may reveal the possible cause of death (to be verified by autopsy). Yet the absence of any bloodsuggests that the victim wasn’t shot where he was discovered. Or was he?

Are we dealing with one or more people who carried the body from another location to the kitchen, or simply a perpetrator who sought to cover his tracks by washing away the blood?

The answer to this question may be revealed through the use of luminol, a reagent which causes chemiluminescence when it comes into contact with blood. Areas previously covered in blood will show up as a blue-green glow with the use of a fluorescent light. But, there is a problem: any number of fluids can react with luminol, including bleach (often used on kitchen floors), saliva, as well as any number of plant and animal proteins.

So how do forensic investigators determine the presence of blood?

The Kastle-Meyer test detects minute traces of heme, which is the principle component of haemoglobin (the molecule in red blood cells that carries oxygen, and unique to all vertebrates and one fish species). Specifically, the Kastle-Meyer test uses phenolphthalein as a chemical indicator of haemoglobin.

A swab dipped in ethanol is gently rubbed over a suspected blood sample and then brought into contact with phenolphthalin. The peroxidase-like activity of hemewill catalyse the oxidation of phenolphthalin into phenolphthalein, which becomes visible as a bright-pink colour. After a positive test a sample is sent to laboratories for DNA testing – to determine whether the blood is of human origin, and whether the blood belonged to the victim.

Should the collected sample belong to the victim, blood spatter patterns may then be analysed to determine the position of both the victim and the perpetrator, as well as other relevant information pertaining to the investigation.

This process is just a small part of the forensic investigation, and is often accompanied by other techniques which includes fingerprint collection and analysis, shoeprint identification (as the perpetrator stepped in the blood while cleaning), fibre analysis, and so on. In the end, however, the aim is always the same: to reveal events as they occurred, and the identity of those involved.