BLOOD PATTERN ANALYSIS
Blood Pattern Analysis
Often found at the scenes of violent crimes, the analysis of bloodstains can provide vital clues as to the occurrence of events. Though bloodstain pattern analysis (BPA) can be a subjective area of study at times and often reliant on the experience of the investigator, the idea that blood will obey certain laws of physics enables the examination of blood at an incident scene and on items of evidence to offer at least an insight into what was likely to have occurred. The successful interpretation of bloodstain patterns may provide clues as to the nature of the offence, the possible sequence of events, any disturbance to the scene that may have occurred, and even the position of individuals and objects during the incident. It may prove beneficial in refuting or corroborating eyewitness accounts. The appearance of a bloodstain can depend on a number of factors, including the velocity at which it was traveling, distance travelled, the amount of blood, the angle of impact, and the type of target onto which it lands.
Single blood drop
A single blood drop typically refer to blood drops that have fallen vertically, whether it be from an injured person or another object, and landed onto another surface. As a blood drop falls perpendicular to a surface it maintains a spherical form until impacting. The size and appearance of this stain will depend on a number of factors. The volume of a single drop of blood will vary depending on the quantity of blood present and the surface area available from which the drop is falling.
Impact Spatter
This type of bloodstain is the result of a forceful impact between an object and wet blood, causing the blood to break into smaller droplets. A greater force will typically produce smaller droplets, with the density of blood drops decreasing moving further away from the initial blood source. The study of impact spatter may provide insight into the relative position of individuals and objects during an incident and the nature of the incident.
Cast-Off Stain
Cast-off bloodstains occur when centrifugal force causes blood drops to fall from a bloodied object in motion. Similarly, cessation cast-off patterns may result from the sudden deceleration of an object. In this instance, the blood flung from a blood-stained object, such as a weapon, may produce characteristic patterns of numerous individual blood drops forming a curved or straight line. If an object is repeatedly moved, each subsequent swing will result in less cast-off as less blood remains on the object. Bloodstains produced in this fashion can be particularly difficult to interpret as there is a great deal of possible variation in patterns produced. However depending on the nature of the motion of the bloodied object, cast-off blood will at least produce relatively linear stains.
Transfer Bloodstains
Transfer or contact stains result when a bloodied surface comes into contact with another surface, transferring blood to that secondary target. The study of this type of bloodstain can prove particularly beneficial in establishing a sequence of events at the incident scene and tracing the movement of objects or individuals. In some cases it may even be possible to establish what object the transfer stain was likely to be caused by, for instance if a particular pattern is produced that can be traced to a blood-bearing object. Similarly, such bloodstains may be left by the hands of an individual, thus opening the possibility of fingerprint evidence.
Examination of Bloodstain Patterns
Various factors must be taken into account in order to successfully interpret a bloodstain. The surface onto which the blood is found may have had an effect on the behaviour and appearance of the stain. Studying the state of the bloodstain may be able to shed light onto how much time has passed since the blood was shed, as over time blood will naturally coagulate (the process by which liquid blood turns into a gelatinous substance through various clotting factors). Furthermore, the extent of drying or coagulation will depend on the quantity of blood present – for instance a single drop will dry significantly faster than a large pool of blood. During this process of coagulation serum stains may be formed, which occur when the serum (liquid portion of the blood) separates.
Bloodstains at an incident scene may not always be visible to the naked eye, either due to low amounts of blood present or an individual cleaning in attempts to remove signs of bloodshed. Despite the use of cleaning reagents or even attempting to cover the stains with paint, detectable traces will generally remain, which can be visualised using various chemicals or specialised light. Although blood will not fluoresce under UV light like some bodily fluids, it will significantly darken, thus enhancing its visibility. Furthermore, certain chemical reagents can be used to visualise latent bloodstains. These tests, such as luminol and phenolphthalein, generally work by reacting with a constituent of blood to produce some kind of chemiluminescence. However it should always be remembered that these chemical reagent tests are often presumptive, meaning that they can only indicate that the stain is possibly blood. In reality, other substances may react with the reagent in the same way.
A lack of a bloodstain can be just as revealing. The absence of blood in a continuous bloodstain is known as a void, and may suggest that something or someone was present in that area when the bloodstain was caused. This could indicate an object present at the time of the incident has been removed from the scene, or an individual (or even multiple individuals) were present in specific locations when blood was shed. Documentation of bloodstain evidence will most typically be carried out using photography, including photographs of the wider scene along with close-up images of particular bloodstains. For more information see: https://aboutforensics.co.uk/bloodstain-pattern-analysis/
Often found at the scenes of violent crimes, the analysis of bloodstains can provide vital clues as to the occurrence of events. Though bloodstain pattern analysis (BPA) can be a subjective area of study at times and often reliant on the experience of the investigator, the idea that blood will obey certain laws of physics enables the examination of blood at an incident scene and on items of evidence to offer at least an insight into what was likely to have occurred. The successful interpretation of bloodstain patterns may provide clues as to the nature of the offence, the possible sequence of events, any disturbance to the scene that may have occurred, and even the position of individuals and objects during the incident. It may prove beneficial in refuting or corroborating eyewitness accounts. The appearance of a bloodstain can depend on a number of factors, including the velocity at which it was traveling, distance travelled, the amount of blood, the angle of impact, and the type of target onto which it lands.
Single blood drop
A single blood drop typically refer to blood drops that have fallen vertically, whether it be from an injured person or another object, and landed onto another surface. As a blood drop falls perpendicular to a surface it maintains a spherical form until impacting. The size and appearance of this stain will depend on a number of factors. The volume of a single drop of blood will vary depending on the quantity of blood present and the surface area available from which the drop is falling.
Impact Spatter
This type of bloodstain is the result of a forceful impact between an object and wet blood, causing the blood to break into smaller droplets. A greater force will typically produce smaller droplets, with the density of blood drops decreasing moving further away from the initial blood source. The study of impact spatter may provide insight into the relative position of individuals and objects during an incident and the nature of the incident.
Cast-Off Stain
Cast-off bloodstains occur when centrifugal force causes blood drops to fall from a bloodied object in motion. Similarly, cessation cast-off patterns may result from the sudden deceleration of an object. In this instance, the blood flung from a blood-stained object, such as a weapon, may produce characteristic patterns of numerous individual blood drops forming a curved or straight line. If an object is repeatedly moved, each subsequent swing will result in less cast-off as less blood remains on the object. Bloodstains produced in this fashion can be particularly difficult to interpret as there is a great deal of possible variation in patterns produced. However depending on the nature of the motion of the bloodied object, cast-off blood will at least produce relatively linear stains.
Transfer Bloodstains
Transfer or contact stains result when a bloodied surface comes into contact with another surface, transferring blood to that secondary target. The study of this type of bloodstain can prove particularly beneficial in establishing a sequence of events at the incident scene and tracing the movement of objects or individuals. In some cases it may even be possible to establish what object the transfer stain was likely to be caused by, for instance if a particular pattern is produced that can be traced to a blood-bearing object. Similarly, such bloodstains may be left by the hands of an individual, thus opening the possibility of fingerprint evidence.
Examination of Bloodstain Patterns
Various factors must be taken into account in order to successfully interpret a bloodstain. The surface onto which the blood is found may have had an effect on the behaviour and appearance of the stain. Studying the state of the bloodstain may be able to shed light onto how much time has passed since the blood was shed, as over time blood will naturally coagulate (the process by which liquid blood turns into a gelatinous substance through various clotting factors). Furthermore, the extent of drying or coagulation will depend on the quantity of blood present – for instance a single drop will dry significantly faster than a large pool of blood. During this process of coagulation serum stains may be formed, which occur when the serum (liquid portion of the blood) separates.
Bloodstains at an incident scene may not always be visible to the naked eye, either due to low amounts of blood present or an individual cleaning in attempts to remove signs of bloodshed. Despite the use of cleaning reagents or even attempting to cover the stains with paint, detectable traces will generally remain, which can be visualised using various chemicals or specialised light. Although blood will not fluoresce under UV light like some bodily fluids, it will significantly darken, thus enhancing its visibility. Furthermore, certain chemical reagents can be used to visualise latent bloodstains. These tests, such as luminol and phenolphthalein, generally work by reacting with a constituent of blood to produce some kind of chemiluminescence. However it should always be remembered that these chemical reagent tests are often presumptive, meaning that they can only indicate that the stain is possibly blood. In reality, other substances may react with the reagent in the same way.
A lack of a bloodstain can be just as revealing. The absence of blood in a continuous bloodstain is known as a void, and may suggest that something or someone was present in that area when the bloodstain was caused. This could indicate an object present at the time of the incident has been removed from the scene, or an individual (or even multiple individuals) were present in specific locations when blood was shed. Documentation of bloodstain evidence will most typically be carried out using photography, including photographs of the wider scene along with close-up images of particular bloodstains. For more information see: https://aboutforensics.co.uk/bloodstain-pattern-analysis/