An Overview of Forensic Blood Analysis

Detecting and identifying bodily fluids at a crime scene is an important part of forensic analysis and can give people an understanding of the events that may have played out during the crime. The presence of body fluids and their location and DNA profile can be used as a part of the investigation and will often result in a conviction.

Body fluid analysis is a complex thing, and while there are numerous techniques that can be used, none are perfect. Alternative Light Sources are one tool that is often used to help people to find stains that may otherwise go unnoticed. This technique can flag up invisible or ambiguous stains and is useful for revealing stains that someone has tried to cover up, for example by wiping the area clean or painting over it. Unfortunately, ALS analysis does not clearly differentiate between certain types of body fluid, and therefore it is not a perfect technique for identifying body fluids, but it is still useful because once the potential evidence is discovered, there are presumptive tests that can be done to determine what the fluids are. If these tests are able to identify what a stain is, then the next step is confirmatory analysis and DNA analysis. Since these tests can be destructive it is important to take care to ensure that there is enough of the substance left to perform multiple analysis.

Analysing Blood

The most commonly found body fluid of interest at the average crime scene is blood. It is primarily made up of water and blood cells, and there are also metabolites, hormones and glucose in the fluid. The importance of blood in a forensic context means that there are numerous techniques used to identify it.

Presumptive Tests

Luminol is perhaps the most well-known presumptive test, and when it is used on a blood sample it will make it glow blue, because of the oxidation of the luminol by the haemoglobin in the blood. Another popular test is Fluorescein, which also relies on oxidation, although the test requires the use of an alternative light source so that the change can be seen. The Kastle-Meyer, or Phenolphthalein, test is one which uses an alkaline solution that turns pink when in the presence of blood.

One test that responds to the presence of heme in blood is the LMG test. This uses a substance which turns green when it comes into contact with heme. There are also some commercially sold tests, such as HemaTRace, Heme Select and ABAcard, which rely on immunological tests.

There are numerous presumptive tests, and different police forces use different tests, often varying even in the same state, let alone country. These tests are presumptive, however, and are not totally conclusive. If they return a positive result then this does not prove that the stain is blood, it simply indicates that it is worth using further testing on the substance in question.

Conclusive Tests

There is a more conclusive test called the Rapid Stain Identification Test which works by detecting glycophorin A. Enzyme-linked immunosorbent assays can be used to identify blood and to differentiate between different groups. In addition to this, there are crystal tests which may help to confirm the presence of blood. The Teichman test uses the formation of hematin, and there is another test called the Takayama test, which relies on the formation of crystals of haemo-chromogen. There are other analytical techniques, such as microscopy, which identify blood cells. Some labs use UV-vis or fluorescence spectroscopy to identify blood with the help of a liquid dispensing machine.

The presumptive tests are invaluable and are great for helping law enforcement to confirm where they should be directing their laboratory resources. These tests are not totally conclusive, however, and they can sometimes react with unexpected substances to give false positive. In addition, small samples can sometimes produce false negatives because of the lack of sensitivity of the tests. For this reason, officers should be trained to identify the right tests for each type of stain based on location, amount and what it is that they suspect the stain contains. This reduces the possibility of errors to a tolerable level.