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Forensische Gerechtelijke Politie West-Vlaanderen

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Abstract Bachelor Project 2019-2020: Comparison of methods for blood detection on dark fabric

For forensic investigators, the main goal when entering a crime scene is to collect a sufficient amount of useful traces, such as blood. Bloodstain pattern analysis and laboratory tests can be used to analyze these traces. The results can then be used as evidence in court.

Not all traces are visible to the naked eye. Sometimes the delinquent has cleaned the crime scene. Other times, bloodstains can be located on dark fabric, making them hard to discern due to the lack of contrast. The latter occurs regularly in the work field. Therefore, it was the goal of this study to compare a few frequently used blood detection techniques, so that crime scene investigators have a clearer vision on which techniques they should use when they are confronted with this issue.

The blood detection techniques that are examined are forensic light sources, tetrabase, Bluestar® forensic, and fluorescein. Each of these techniques are tested on ten different kinds of fabric. On each item, an undiluted drop of blood is applied. All techniques, besides the forensic light sources, are also tested on larger pieces of fabric, to approximate reality.

The results obtained with the forensic light sources vary from fabric to fabric. This was not the case with the other tests. Tetrabase is a preliminary test for blood, but is here also used to determine latent bloodstains. It has the advantage that it can be used under normal lighting (no need to darken the environment). Because the bloodstain is absorbed on a swab or a piece of paper, the dark hue of the fabric is of no importance.

Fluorescein and Bluestar® forensic are both very easy to apply, but both techniques require a dark environment. A positive reaction with fluorescein is bright and long-lasting, but depending on the condition of the fabric, excessive background fluorescence can interfere with the observation. Therefore, it is necessary to observe the fabric under blue light, before applying fluorescein to determine whether it can be used on the sample. Not all reactions with the bloodstains were captured on camera, due to interfering background fluorescence.

A positive reaction with Bluestar® forensic is more subtle and fades away fast, however the reaction is easily captured on camera and all bloodstains are discovered. The necessity of total darkness makes it uncomfortable to work, but the ease of application makes up for this. Mixing the Bluestar® forensic solution is easier than the fluorescein solution, which required adding hydrogen peroxide just before use.

In conclusion, when black fabric needs to be examined for bloodstains, it has to be observed under forensic light sources first. If there is a visible stain, tetrabase can be used to determine the presence of blood. Tetrabase can also be used to detect latent bloodstains, but so can fluorescein and Bluestar® forensic. All three techniques are reliable. Personal preference may be the deciding factor, however when using fluorescein, it is necessary to evaluate the sample under blue light first, to check the background fluorescence. 


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