TERMINOLOGY IN BPA
Bloodstains can be grouped into three main classes: passive, transfer, and projected or impact stains. Passive bloodstains include clots, drops, flows, and pooling. Transfer bloodstains include wipes, swipes, pattern transfers, and general contact bloodstains. Finally, spatters, splashes, cast-off stains, and arterial spurts or gushes are examples of projected or impact bloodstains. Other patterns include fly spots, voids, and skeletonized stains. A wipe stain is created when an object moves through a pre-existing bloodstain. An example would be the stain resulting from a clean rag being moved through a blood pool on a floor. In contrast, a “swipe stain” is the transfer of blood onto a target by a moving object that is itself bloodstained. Blood-soaked clothing being dragged over unstained vinyl flooring would result in a swipe, not a wipe. An easy way to remember this difference is that an object that is stained creates a swipe; an unstained object makes a wipe (no stain, no “s”). Spatter is a technical term in BPA that describes a stain that results from blood hitting a target. Two types of spatter are recognized. The first, forward spatter, results when blood droplets are projected away from the item creating the impact, such as a hammer. Back spatter, in contrast, is caused by droplets being projected toward the item; in general, back spatter will be lighter and the stains smaller than forward spatter (Figure 1). Note that the word “splatter” has no technical meaning in BPA and should not be used.
FIGURE 1 Forward spatter results from droplets being projected to the front of the object hitting the blood; back spatter is the opposite. Cast-off stains result from blood being flung from a bloodied object; note that cast-off stains can occur behind the item as well—for example, on the back of the assailant’s shirt.
Cast-off stains are the result of blood being flung or projected from a bloody object in motion or one that stops suddenly (see Figure 1). Cast-off stains are linear and reflect the position of the person moving the bloody object. If a criminal bludgeons a victim with a baseball bat, as the criminal’s arm comes back to swing again, any blood on the end of the bat will be projected by centrifugal force in an arc. Remember that cast-off stains can arc directly behind the object and land, for example, on the back of the assailant’s shirt. The blood flowing through the arteries is under high pressure. When an artery is breached while the heart is pumping, blood will spurt or gush from the wound, as depicted in Figure 2. Arterial spurts/gushes can vary due to the pumping action and variable pressure of the blood as it exits the wound, producing a zig-zag, up-and down pattern. Fly spots are stains resulting from fly activity—and may mimic other relevant BPA patterns. Flies may regurgitate and defecate when consuming blood at a crime scene, and these spots, as well as small amounts of blood that are tracked, can be confusing or misleading. Voids are an indicator that some secondary object came between a blood spatter and the final target; this leaves an outline or “shadow” on the final target, illustrated in Figure 3. Voids are important clues about items that may have been moved or discarded after an attack but were present during the criminal process.
FIGURE 2 The arterial spurt pattern from the victim (a) can be seen in detail on the round ottoman (b).
FIGURE 3 Voids indicate that some object came between a blood spatter source and the final target. The outline or “shadow” is an important clue that some item that was present during the incident may have been moved or discarded afterward.
FIGURE 4 The “tail” of the stain indicates direction; the angle from a 0° reference point shows the directional angle. The satellite droplets are small “splashes” from the parent stain.
As a stain dries, the edges and borders dry first due to surface effects. If the bloodstain is wiped, these dried areas, called skeletonized stains, remain behind. Skeletonized stains retain the size and shape of the original stain and indicate the passage of time. Figure 4 shows some of the basic measurements for BPA. The angle of impact is the acute angle created by the intercept of the target with the droplet’s vector. This is different from the direction angle, which is the angle between the long axis of the stain and a standard reference point, usually 0° vertical. The directionality demonstrates the vector of a droplet when it hits the target; the tail points in the direction of travel. Satellite droplets are small amounts of blood that detach from the parent stain and “splash” onto a surface.
