Forensic blood analysis is an invaluable tool for detecting and identifying bodily fluids at crime scenes. It can be very useful to help law enforcement determine why events might have occurred and who was involved with them. Concerns such as the presence of body fluids, where they are, and any DNA found in them could supply vital information that will help to solve the crime.
Body fluid analysis can sometimes be a lengthy process. There are visualisation techniques that can be employed at the scene of an incident, to help to locate potential body fluids. For example, alternative light sources can be useful. An ALS test, for example, makes use of the different wavelengths of light so that they can show up stains which might otherwise be ambiguous or even invisible. In some cases, it can highlight stains that were covered with paint or ones that have been wiped clean.
Once those stains have been revealed, it is usually possible to differentiate between the fluids that are found, and analysts can use presumptive tests using liquid dispensing machine by Flux Pumps to identify the stain. At the moment, there is not one single method that will work to determine the type of a stain, so it may be necessary to use more than one technique depending on the kind of biofluid and the confidence with which the initial analysis was performed.
Blood is the most common fluid that is found at a crime scene and is usually of great interest. It is made up of water, blood cells, hormones, metabolites, glucose, minerals and proteins. The importance of blood as a piece of forensic evidence means that there are numerous tests developed for it. Initially, the tests may be used to confirm with a degree of certainty that the stain is indeed blood. There is one presumptive test which uses the blue luminescence of blood (from the oxidation of luminol in haemoglobin) to determine that the substance is blood. Fluorescein uses a similar idea, based on the how fluorescin is turned into fluorescein when it is oxidised. This requires an alternate light source in order for the analyst to see the change properly.
Another option is the Phenolphthalein test, which is sometimes referred to as the Kastle- Meyer test. It uses an alkaline solution to cause the blood to take on a pink colouring. Leucomalachite green, or LMG, is sometimes used to test for blood as well. When the test is positive, it turns a distinctive green colour, as the substance reacts to the presence of Heme.
There are several immunological tests that are widely available on the market, such as ABAcard, Heme Select and HemaTrace. The tests that are performed differ regionally, and even between different police forces. The tests are all presumptive, and even if positive it may be necessary to use other tests to confirm the identity of the bodily fluid that was found.
There are some other options, such as the Rapid Stain Identification TEst, which is an antibody-based method which can help to confirm the presence of blood-based glycophorin A. It is also possible to use Enzyme-Linked Immunosorbent Assays (ELISA) to differentiate between blood types and to identify blood. There are crystal tests, too, which are based on the formation of Hematin, and can confirm that a substance is indeed blood. The Takayama test is based on the formation of haemochromogen, for example. Other techniques which can be used include microscopy to identify the presence of blood cells, and UV-=vis or Fluorescence spectroscopy.
When working in a forensic context, presumptive tests are the starting point because they offer an easy and fast way to perform an in-situ assessment of the fluids in question. These can then lead to laboratory-based tests if necessary. The main concern with any presumptive test is that it is not particularly specific. The tests can give false positives, and in some cases false negatives too. This is why it is so important to back them up with confirmatory tests and to exercise caution when working with small samples because many tests are quite destructive to the substance being examined.