Bloodstain pattern analysis (BPA) is a part of Crime Scene Investigation related to any crime which involved blood. Blood is one of the most commonly recovered and important piece of evidence used in crime scene investigation. Blood stain analysis is used to interpret the bloodstains at a crime scenes to draw conclusions about the nature, timing and other details of the crime in order to recreate the actions that caused the bloodshed.
To become an expert in Interpretation of bloodstain pattern, one must first study the behaviour of blood under various conditions.
What is Blood?
Red blood cells, white blood cells, platelets, and plasma are among the many components that make up blood. Human blood volume varies depending on age, gender, weight, body type, and other factors, but an adult’s blood volume is roughly 60 ml per kg of body weight.
Blood circulates throughout our bodies, delivering vital nutrients and oxygen to the body’s cells. It also moves metabolic waste products away from the some cells.
How Blood Stain Pattern formed?
Water makes up the majority of blood. An expert can better understand blood behaviour by understanding water. Blood, like water, has surface tension because individual particles within a drop are attracted to one another. Blood’s cohesive property tends to keep droplets together.
A droplet of blood may or may not spatter when it comes into contact with a surface. It will, however, leave a bloodstain. A falling droplet has a nearly spherical shape. When a droplet collides with a surface, the lower edge deforms and blood is forced outward to form a border. Patterns formed by the borders of blood droplets at a crime scene are clues that aid in the investigation.
A crime scene investigator determines the location, distribution, and appearance of bloodstains after a crime has been discovered.
While evidence of blood spatter can be helpful to investigators, it is not perpetually available. Some surfaces, such as fabrics, soil, etc. absorb blood and destroy the shape of the droplets. Many outdoor surfaces are waterproof, so blood won’t stick and in that case it cannot be interpreted.
But in some cases When a drop of blood hits the target’s surface, it forms a spot with or without splashes. The size of the blood stain is determined by the amount of blood. The overall shape of the stain depends on the angle at which the drop of blood falls on the surface as well as the speed at which the blood drop is moving. The finer features of the blood stain shape are varies according to the texture of the target’s surface.
Glass and metal are very tough and easy surfaces that most usually leaves rounded stains. This pattern is because of the fact that clean substances provide much less floor place to deflect the liquid. Highly textured material, like linoleum, cardboard, or news paper, produces stains with jagged edges. Blood drops falling on a textured target may also display scalloping or spines extending from the droplet.
How to collect Blood Stain Pattern?
Bloodstain samples can be collected for further analysis by cutting stained surfaces or materials, and then photographing the stains, and drying and packaging stained objects. If cutting and collecting of stains is not possible then high quality photography works.
Tools Used in Blood Pattern Analysis
Following Tools are used to determine area of convergence and area of origin:
- Elastic strings
- Mathematical equations (tangent trigonometric function)
- Computer software programs (such as BackTrack™ or Hemospat)
- Limiting angles method, which examines the physical evidence to exclude angles from analysis.
How to Analyze Blood Stain Patterns?
Analysts utilise the direction and angle of the splatter to determine the locations of convergence (the start of the bloodshed) and origin to aid in the reconstruction of events that resulted in bloodshed (the estimation of where the victim and suspect were in relation to each other when bloodshed occurred).
Investigators often use thread to draw straight lines across the long axis of individual drops, following the angle of impact along a flat plane, such as the floor or wall where the drops are detected, to find the location of convergence. Investigators can determine where the victim was when the drops were made by following the lines to where they intersect.
Investigators implement a similar procedure to determine the origin area, but they additionally put height estimations also. This generates a 3-D model of the victim’s location at the time of the drops. For example, if the point of origin is barely two feet above the point of convergence on the floor, the analyst might assume the victim was lying or sitting there. If it’s five feet above the ground.
- Miller, M. T., & Massey, P. (2019). Chapter 8 Reconstruction Activities. In: The Crime Scene. 2nd ed; 179-227. doi:10.1016/C2016-0-04393-0.
- Holowko, E., Januszkiewicz, K., Bolewicki, P., et al. (2016). Application of multi-resolution 3D techniques in crime scene documentation with bloodstain pattern analysis. Forensic Science International 267; 218-227. doi:10.1016/j.forsciint.2016.08.036.
- Attinger, D., Liu, Y., Faflak, R., Rao, Y., et al. (2019). A data set of bloodstain patterns for teaching and research in bloodstain pattern analysis: Gunshot backspatters. Data in Brief 22; 269-278. doi:10.1016/j.dib.2018.11.075.