Summary
Highlights
Blood spatter evidence can reveal crucial information about a crime, including the type of weapon used, the number of times a person was attacked, the suspect's handedness, victim/suspect location and movement, specific injuries, time since the crime, and even how long it took for the victim to die.
To detect blood evidence, especially if it's not visible or has been cleaned, several methods are used. UV light sources can reveal traces of blood and body fluids. Presumptive tests, like the Kastle-Meyer test, react to hemoglobin, turning phenolphthalein pink in the presence of blood. Luminol can make blood glow, even if cleaned, but it may destroy other evidence. Fluorescein is a similar spray solution that glows green under UV light.
Key terms in blood spatter analysis include 'origin' or 'source' (where the spatter came from), 'angle of impact' (the angle at which the droplet hits a surface), 'parent drop' (the main blood drop), 'satellite spatters' (smaller drops detaching from the parent drop), and 'spines' (pointed edges on the parent drop indicating direction).
There are three main types of blood stain patterns: Passive (formed by gravity, like drips), Projected (resulting from force, such as being hit or arterial spurting), and Transfer/Contact (when a blood-stained object touches another surface, like bloody footprints or smears).
When blood falls, it initially forms a teardrop shape then a sphere due to surface tension. The appearance on impact depends on the surface: a smooth surface yields a smooth, circular pattern, while an irregular surface creates satellite droplets and spines. The size of blood drops can indicate the height from which they fell; larger drops generally mean a higher fall, though this reaches a maximum at about 6-7 feet due to terminal velocity.
The size and quantity of blood drops can indicate the type of wound. Fine mist spatter with many small droplets often suggests a gunshot, while larger droplets indicate a low-velocity impact from a blunt object. Voids in blood patterns suggest an object was present during the spatter event and has since been removed.
Blood drops falling at a 90-degree angle are circular and don't indicate direction. However, if blood hits a surface at an angle, the spines and the shape of the parent drop, particularly what’s called 'forward cast off' or smaller droplets ahead of the main drop, will point in the direction of movement. This occurs because the blood retains momentum in the direction of travel.
Lines of convergence help pinpoint the origin of blood spatter. By drawing lines through the central axis of at least two blood drops, forensic analysts can find where these lines intersect, indicating the 'area of convergence'—the spot from which the blood originated or where the bleeding person was located.
In a gunshot wound, spatter is visible from both the entrance and exit wounds. While some spatter may come from the front upon entry, the majority of the blood spatter, often a 'fine mist spatter,' will be observed from the exit wound.