The recognition and analysis of bloodstain patterns can yield useful investigative information. The general role of the Bloodstain Pattern Analysis in a criminal investigation is to assist in the reconstruction of those events of an alleged incident that could have created the stains and stain patterns present at a crime scene, on items of physical evidence recovered from that scene and on items of clothing that were present at the crime scene. Information that may be gained with bloodstain pattern analysis include, for example, the position of the individual when the blood was deposited (sitting, standing, etc.), the relative position of individuals at the time of bloodshed, the possible type of weapon used as well as possible mechanisms that could have produced the blood staining on a surface .
Bloodstains range in both amount of blood and type of pattern—from pools of blood around a body to obvious spatter patterns on the walls to microscopic drops on a suspect’s clothing. The shape of the bloodstain pattern will depend greatly on the force used to propel the blood as well as the surface it lands on. Forward spatter from a gunshot wound will typically form smaller droplets spread over a wide area, while impact spatter will form larger drops and be more concentrated in the areas directly adjacent to the action.
Stains that overlap or come from multiple sources present challenges to analysts, but often reveal valuable details about the crime. Overlapping stains may obscure pattern details, but can provide information on the force, timing and instrument used. In the case of multiple victims, analysts will often use DNA profiling to determine whose blood is included in a given pattern, helping to estimate the locations of the victims in relation to each other and the perpetrator(s) .
Crime Scene Blood Stain Pattern Analysis:
Bloodstain pattern analysis is the application of scientific knowledge to the examination and the interpretation of the morphology, the sequence, and the distribution of bloodstains associated with a crime. These analyses may determine the sequence of events; the approximate blood source locations; the positioning of the victim; and the position, the intensity, and the number of impacts applied to the blood source. They also can assist in the determination of the manner of death and can distinguish between accidents, homicides, and suicides. Bloodstain pattern analysis provides critical information for crime scene reconstructions in violent crime investigations.
Formation of Blood Stains:
The formation of a blood droplet is a complex event that is influenced by viscosity, surface tension, cohesion force, and gravity. Blood is viscous, and blood viscosity is a measure of the blood’s resistance to flow. The viscosity of blood is approximately five times greater than that of water. During the formation of a drop of blood, blood leaks out from a blood source. The surface tension of the blood causes it to hang from the opening of a blood source and to form a pendant drop of blood. The molecules of a blood drop are held together by the cohesion force to maintain the shape of a blood drop. Surface tension causes liquids to minimize their surface. As a result, the formed blood drop is spherical. As the volume of the drop gradually increases and exceeds a certain size, it detaches itself and falls. The falling drop is also held together by surface tension. A falling blood drop is influenced by the downward force of gravity acting on the drop and the air resistance that acts in the opposite direction as the drop is in motion.
When a bloodstain lands on a surface, the shape and the size of the bloodstain is affected by the texture of the target surface. Bloodstains that land on porous or rough surfaces usually have more distortion around the edges of the stains than those that land on smooth surfaces. A comparison of blood dropped onto different textures of target surfaces.
Types of Bloodstain Patterns:
Bloodstain patterns can be classified into three basic categories:
- Passive Bloodstains: A passive bloodstain is formed due to bleeding from wounds, and the blood is deposited on a surface by the influence of the force of gravity alone.
- Transfer Bloodstain: A transfer bloodstain is a bloodstain resulting from contact between a blood-bearing surface and another surface.
- Projected Bloodstain: A projected bloodstain is formed when a volume of blood is deposited on a surface under a pressure or a force that is greater than the force of gravity .
How is Blood Stain Evidence Collected?
Bloodstain samples can be collected for Bloodstain Pattern Analysis by cutting away stained surfaces or materials, photographing the stains, and drying and packaging stained objects. The tools for collecting bloodstain evidence usually include high-quality cameras (still and video), sketching materials, cutting instruments and evidence packaging.
Documentation of Bloodstain Evidence:
The most frequently used method of capturing bloodstains is high-resolution photography. A scale or ruler is placed next to the bloodstain to provide accurate measurement and photos are taken from every angle. Video and sketches of the scene and the blood stains is often used to provide perspective and further documentation. This is commonly done even if stained materials or objects are collected intact.
Sampling of Bloodstain for DNA Profiling:
Analysts or investigators will typically soak up pooled blood, or swab small samples of dried blood in order to determine if it is human blood and then develop a DNA profile. This becomes critical when there are multiple victims. DNA profiling may also indicate whether the perpetrator was injured during the attack, as in the case of two DNA profiles found at a scene with only one known victim.
Whenever possible, analysts or crime scene investigators try to collect the evidence intact. This may require removing a section of a wall or carpeting, furniture, or other large objects from the crime scene and sending them to the laboratory for analysis. Items that cannot be removed, such as a section of concrete flooring, will be thoroughly photographed and documented.
How & Where Analysis is performed?
Bloodstain analysts use established scientific methods to examine bloodstain evidence at a crime scene including information gathering, observation, documentation, analysis, evaluation, conclusion and technical (or peer) review.
It is performed in two phases: pattern analysis and reconstruction.
- Pattern Analysis looks at the physical characteristics of the stain patterns including size, shape, distribution, overall appearance, location and surface texture where the stains are found. Analysts interpret what pattern types are present and what mechanisms may have caused them.
- Reconstruction uses the analysis data to put contextual explanations to the stain patterns: What type of crime has occurred? Where is the person bleeding from? Did the stain patterns come from the victim or someone else? Are there other scene factors (e.g. emergency medical intervention, first responder activities) that affected the stain patterns?
To help reconstruct events that caused bloodshed, analysts use the direction and angle of the spatter to establish the areas of convergence (the starting point of the bloodshed) and origin (the estimation of where the victim and suspect were in relation to each other when bloodshed occurred).
Tools used to determine area of convergence and area of origin include:
- Elastic strings and protractors
- 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 (e.g. if blood is found on the underside of a desk or table, analysts know that at least a portion of the spatter-producing event took place below the height of the desk or table.).
Chemical Enhancement of Bloodstain Evidence:
Many chemical reagents react with blood to exhibit a color, a chemiluminescent light, or a fluorescent light. These tests are extremely sensitive and thus are used as chemical enhancement reagents for detecting bloodstains. For bloodstain pattern analysis, the enhancement reagent is primarily used for detecting latent bloodstains such as diluted bloodstains that are visible on enhancement. A commonly used chemical reagent is luminol, which can be used for locating bloodstains at the scene. Other reagents such as phenolphthalein, leucomalachite green, and tetramethylbenzidine are not often used as enhancement reagents but rather as presumptive tests for blood. The positive reactions of all these reagents indicate the presence of blood .
- “Bloodstain Pattern Analysis” [Online] (https://dps.mn.gov/divisions/bca/bca-divisions/forensic-science/Pages/forensic-programs-crime-scene-bpa.aspx) Accessed on 25/9/2018.
- “Bloodstain Pattern Analysis: How it’s done” [Online] (http://www.forensicsciencesimplified.org/blood/how.html) Accessed on 25/9/2018.
- Li, R. (2015), “Forensic Biology (2nd Edition)”