There are many different factors to consider when analyzing blood stain patterns. The first thing that an investigator wants to determine is what kind of pattern is being presented. Blood stain patterns can be presented as:
• Drip Stains/Patterns
– Blood Dripping into Blood
– Splashed (Spilled) Blood
– Projected Blood (with a syringe)
• Transfer Stains/Patterns
• Blood Spatter
• Shadowing/ Ghosting
• Swipes and Wipes
• Expiratory Blood
When an investigator is analyzing drip stains/ patterns, blood spatter, shadowing/ghosting, and expiratory blood there are different factors they have to look at, these factors include:
– Whether the velocity of the spatter is low, medium or high
– The angle of impact
A low velocity spatter is usually four to eight millimeters in size and is often a result of dripping blood after a victim sustains an injury such as a stab or in some cases a punch. For example, if a victim is stabbed and then walks around bleeding, the drops of blood that are left behind are low velocity. The low velocity drops in this example are passive spatters. Low velocity spatter can also result from pools of blood around the body and transfers. A medium velocity spatter is a result of a force anywhere from five to a hundred feet per second .This type of splatter can be caused by a blunt force such as a baseball bat or an intense beating. This type of spatter is usually no more than four millimeters. This type of spatter can also be a result of a stabbing. This is because arteries can be hit if they are close to the skin and blood can spurt from these wounds. This is classified as projected blood. A High velocity spatter is generally caused by a gunshot wound but can be from a wound from another type of weapon if enough force is used.
Once the type of velocity is determined it’s important to determine the angle of impact. These two factors are important to find so that it’s possible for a point of origin to be determined. A general observation that can be made by investigators about the angle without any calculations being involved is that the sharper the angle, the longer the “tail” of the drop is. The angle of impact is determined by dividing the width by the length of the drop. Once the angle is determined investigators then take the arcsine (inverse sine function)of that number and then use stringing ( the use of strings to chart the trajectories of all the blood droplets in the air) to determine the point of origin ( where the stings converge).