Blood Pattern Analysis: Drops, Stains, Spatter, Cast-Offs

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What can forensics tell from the pattern of blood at a crime-scene?

Known as BPA, blood pattern analysis invaluable tool for any crime-scene investigator, as although (as we saw in Mini Mile #9) although you can semi-successfully clean-up a crime-scene, you can’t tamper with the laws of physics. So by understanding at what speed, height, distance and angle the blood travels, and how it impacts with a surface, that can help an investigator to establish what crime took place, the sequence of events, the position of those involved, and to confirm or deny an eye-witness’s, victim’s and perpetrator’s description of the events.

There are eight primary blood patterns.

#1 A Single Drop: A single drop of blood, falling vertically and forming an evenly spread spherical shape shows that the victim was standing and motionless at the time they bled, with the blood stain being more tear-drop shaped, the greater the victim moves. Although the higher the blood falls the larger the stain should be, the width of the stain can’t tell you what height it fell from, as this also depends on the quantity of the blood and the surface it fell onto, as absorbent surfaces (like carpet or clothing) create smaller stains, but flat or rough textured surfaces (like tile, wood and stone) can distort the shape and create its own spray, known as satellite stains.

#2 Impact Spatter. When a victim is violently struck, the force of the blow results in the expulsion of blood droplets from the injury, at speed, from a height and at an angle, but as the blood droplets are propelled through the air, they will disperse into even smaller droplets and become more scattered the further they are propelled from a single point of impact. And as before, each droplet will land in a specific way (from a tear-drop shape to a long thin streak) which tells where the point of impact was. If the impact spatter is interrupted, this will show where a person or object was at that moment as instead of blood-droplets, there will be a void.

#3 Cast-Off Stains. These occur when a blood-stained object is cast aside, just like when you shake-off the rainwater from a soaked umbrella, the speed and angle of your wrist flicking off the excess water will produce a very distinctive pattern. If a perpetrator throws aside a bloodied knife, the excess blood cast off the object will produce a pattern of its own, usually a linear or curved shape, giving you a rough location, and the smaller and less dense the impact spatter, the further away it was cast.

#4 Transfer Bloodstains. This is when a bloodied surface comes into contact with a second surface (whether clothing, a person or weapon) that creates an unnatural smear or smudge, which unlike a single drop, impact spatter or cast-off stain is very unique, showing you what speed, height and direction of transfer occurred, and like impact spatter, the further the smear, the less dense the blood stain. And if the object was stationary, sometimes an imprint of the original object can be left on the secondary source; such as fingerprints, pattern of the material, and also fibres.

#5 Projected Pattern. Sometimes called arterial spray, this is caused by pressurised blood discharging from a tear or rupture of one of the body’s main arteries (carotid, radial, femoral, brachial, temporal and the aorta, although spray from the aorta is less likely, owing to its position in the chest cavity). Like impact spatter, the distance from the victim to the surface is defined by the size and density of blood droplets in the scatter pattern, but with the ruptured artery still pumping blood at high pressure, the impact spatter will be expelled in episodic spurts, which will continue well after the victim has ran, fallen or been moved, with the amount of blood, dependant on the size of the wound.

#6 Pool Stains. These occur when blood accumulates onto a surface from a wound or ruptured artery, forming a pool of blood. If they have accumulated and formed satellite stains (smaller droplets which have splashed outside of the pool) or a trough shape in the centre, this can suggest blood has dripped steadily and continually from a height. A flat accumulation suggests they dripped from a lower height. And an unusual void in the blood pool can suggest the victim was motionless when they were bleeding, but has since moved or had been moved. The quantity of the blood can also suggest how much blood was lost and whether they were alive or dead at the time.

#7 Insect Stains. For flies, a bloody crime-scene is like a buffet, and as they feed on blood and human tissues, they also excrete small circular stains, which appear to be blood spots, but are actually the regurgitation and excretion of food, known as flyspeck. Flyspeck and the presence of insects can help determine a time of death, as a flies biology is as accurate as physics.

And finally, #8 Expiration Stains. These occur when the victim has an injury to his/her mouth, lungs or respiratory tract, and being diluted by saliva and mucus, they are expelled from the mouth as a fine mist, but (unlike impact spatter) they can only be seen if the victim’s mouth/nose is on or near another surface. Expiration stains can help determine if and when the victim was last alive.

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If you found this interesting? Check out the Mini Mile episodes of the Murder Mile UK True-Crime Podcast, or click on the link below to listen to an episode.

Michael J Buchanan-Dunne is a writer, crime historian, podcaster and tour-guide who runs Murder Mile Walks, a guided tour of Soho’s most notorious murder cases, hailed as “one of the top ten curious, quirky, unusual and different things to do in London”, nominated "one of the best true-crime  podcasts at the British Podcast Awards 2018", and featuring 12 murderers, including 3 serial killers, across 15 locations, totaling 50 deaths, over just a one mile walk