Introduction to Bloodstain Pattern Analysis

One of the techniques and analyses used to assist investigators is bloodstain pattern analysis (BPA). Bloodstain pattern analysis refers to the analysis of the size, shape and distribution of bloodstains as a method of determining the mechanism or activity which produced them. There is different information contained within bloodstains that can be interpreted – size, shape, location and even the pattern of the stains. BPA information and results cannot generally be used alone rather it requires additional information from an autopsy and deoxyribonucleic acid (DNA) analysis to be useful. BPA is a discipline that involves knowledge from a range of subjects including biology, mathematics and physics.

Education and Training

There are several paths that one can take to become a bloodstain pattern analyst. There are analysts with different levels of education as there are no universal requirements of educational qualifications at this time. Generally a strong background in biology, chemistry, or physics, many have degrees in areas of forensic science or forensic medicine. However, many are crime scene investigators, detectives or technicians, which have experience and minimal science background. In such cases, college-level courses in mathematics – specifically geometry, and trigonometry as well as basic physics can be helpful.

A basic bloodstain pattern analysis course is also advised, it is a minimum of 40 hours and includes theory as well as practical aspects including case study analysis and laboratory experiments. Advanced courses are available that further focus on specific areas, computer analysis and digital imaging, examination of evidence (clothing, footwear etc.), and include mock crime scene and some may include mock trials. However, these courses do not make one a qualified analyst, it takes 3 years of experience with crime scene analysis and examination of evidence as well as experimentation and the International Association for Identification (IAI) requires a minimum of 240 hours of training in a related field of study (i.e. crime scene investigating, photography). The IAI hosts certifications as well as recertification in several areas including in BPA, it is also a professional organization that one can apply to become a member. To maintain status as a qualified proficient analyst it is important to continue education through attending seminars and conferences as well as rigorous testing and evaluation usually on an annual basis, including annual proficiency tests on techniques used by the analyst.

The International Association of Bloodstain Pattern Analysis (IABPA) is a professional organization that hosts annual conferences and maintains a list of training courses available in several countries. To become a member the successful completion of the basic course is required as well as a recommendation from another member. Although there is also a membership available to those at the general interest level which includes those who have not yet completed the basic course.

Crime Scene Reconstruction

The results and interpretations produced from BPA can be helpful in cases when the manner of death is in question and can be important with crime scene reconstruction. The resulting interpretations of the BPA can help investigators by supporting and/or refute assumptions in regards to the case. Assumptions could be with regards to the type of event, sequence of events, witness or suspect statements. The BPA is important to determine the locations and positions of the victim and the assailant(s) throughout the event. It can be used to determine the location of the victim; standing, sitting or on the floor within the crime scene, or if there was movement within the crime scene, in such cases determining such information could help in the location of other evidence. The interpretations can also help in determining the weapon used, and the amount of force used, as it can narrow down options, dependent on the patterns that are observed.  The bloodstains of patterns can be measured to determine direction and distance from the source of the blood. For crime scene reconstruction to be practical and constructive the process depends on the data available, not only the quantity but the quality and, hopefully, pertinent questions can be tackled. For bloodstain pattern analysis here are several different patterns that can be created from a bloodletting event.


The conventional method to classify blood stains was based on the correlation between the velocity (speed and direction) that affect the characteristics of the resulting stains and size. The three categories are low-velocity impact blood spatter, medium velocity impact blood spatter and high-velocity impact blood spatter. These classifications were constructed on the theory that the size of the stain is inversely proportional to the force applied. Low-velocity impact blood spatter are created with low amounts of force (up to 5 feet/second) and result in bloodstains 4mm (5/32”) or larger. Medium velocity impact spatter are created with an amount of force within the range of 5 feet/second up to 25 feet/second and result in bloodstains that are generally between 1mm and 3mm (3/64” and 1/8”). Stains within this category are generally related to beatings and stabbing events. High-velocity impact blood spatter is created when the amount of force exceeds 100 feet/second and the resulting bloodstains are usually less than 1 mm (3/64”) and are normally associated with gunshot injuries.

Revised Classifications

Current method of classification does not reflect as much upon size or amount of force but rather the terminology is derived from the both the physical appearance of the pattern and the mechanism in which the patterns are created. The analyst must have knowledge of the mechanisms in which bloodstains and the patterns are created in order to be able to assess crime scene evidence. However, the first step is to classify based on the appearance of the stain. Then with case details, it can be determined which mechanism(s) the patterns were created. Based on the appearance of the stain it can be classified into three categories; passive, spatter, or altered. Passive or gravity bloodstains are those that appear created without any outside force except gravity and friction. Spatter bloodstains are associated with an outside force, as well gravity and friction, and show directionality, and vary in size. Altered bloodstains are those that show the blood has undergone physical or physiological changes. Within each of the three categories are subcategories that are mechanism-based to further categorize a bloodstain. With this system, shown in Figure 1, it allows for more naturally occurring overlap to be included in analysis., since there can be some degree of overlap between the classifications with more than one mechanism occurring, referred to as a complex pattern.


Figure 1: Classifications of Bloodstain Patterns



Passive bloodstains are subcategorized as transfer or contact, drop(s), flow and large volume. Transfer or contact stains is when a stained item deposits blood onto a surface as a result of contact. If during the contact movement occurs it is referred to as a swipe pattern as seen in Photo 1.


Photo 1: blood covered gloved hand transferring the blood to a surface, creating a handprint contact stain, with movement to the right of the image


Blood drop patterns can be a single drop or several in an area, but not overlapping. These are free falling towards the surface allowing them to keep a spherical shape, as seen in Photo 2. However, if the surface is not flat as in the photo, a slightly different patterns can emerge based on the texture of the surface, different textures include, carpet, cardboard, tile, wood or cotton. A trail of blood drops would show movement within the crime scene and could show directionality, although not in all cases as seen in Photo 2.


Photo 2: two non-overlapping drops of blood on a horizontal surface


A flow pattern is when a volume of blood present on a surface that moves in one ormore directions as a result of gravity. An example of a single flow pattern would be when blood hits a wall and falls downwards creating a “drip” like pattern. If there aremultiple flow patterns on a single surface it shows that the surface was moving and thus the blood moved as a result of gravity shifting. Such as an injured victimmoving, such as trying to get up or falling to the ground.


Photo 3: two areas of fresh blood moving in a downwards direction on a vertical surface


Free falling large volumes of blood are further sub-categorized into splash, saturation or pooling patterns. Splash patterns occur when the blood falls from a distance of at least 4 inches. Saturation holds no specific patterning but rather occurs when a large volume of blood soaks into a surface, this is most common with fabrics and absorbent materials, including pillows or carpets. Pooling, again, holds no specific pattern but rather is the accumulation of blood on a surface, following its shape.


Spatter is also the blood stains that show directionality and vary in size and are generally associated with a source of blood being subjected to external force(s). Spatter bloodstains are subcategorized as impact mechanisms, secondary mechanisms, or projectile mechanisms. Impact spatter is the result when an object comes into contact with blood. Impact mechanisms can be further sub-categorized as gunshot, beating/stabbing, or industrial and is similar to the original classification methods. Impact spatter is classified as such in conjunction with case details as it is important to note that other methods can produce similar sized staining. For example, spatter produced from blunt force or sharp force trauma may overlap that produced from gunshot trauma, to classify the use of autopsy reports should be considered. With gunshot wounds, there are a few types of spatter that can be present referred to as forward spatter, back spatter and the drawback effect. Forward spatter is when the pattern of drops is directed away from the force causing the spatter; associated with exit wounds. Back spatter is when the pattern of drops is directed towards the force causing the spatter; associated with entrance wounds. The drawback effect is the presence of blood within the barrel of a gun which has been drawn back into the muzzle of the gun and only occurs when the source of the blood is very close to the gun at the time of the firing of the projectile.


Photo 4: spatter produced when a hockey puck is dropped on a small amount of blood on tile

Photo 5: spatter produced when blood on top of a hockey puck is struck with a hammer, simulating blunt force


Secondary spatter is a result of an event other than the blood loss event and refers to satellite spatter. When the blood contacts another surface or additional blood on the surface. The parent stain, the stain from which satellite stains originate, has a distribution of spatter stains surrounding it, one example is drip patterns. Drip patterns are where drops hit an already existing blood stain on a surface as shown in Photo 6.


Photo 6: multiple drops of blood hitting the same spot with an existing stain creating satellite spatter, with the parent stain being larger and darker in color


Projectile spatter is subcategorized into cast-off, arterial and expirated patterns. Cast off patterns are from blood being released from an object in motion (usually linear configuration). Such as after the initial strike the weapon is brought back and blood is released from the object. The stains are normally elongated and show directionality. Cast off patterns usually overlap with impact spatter as both are associated with beating events. Arterial bloodstain patterns that occurs when damage is done to an artery, as the pressure is released, it usually appears like a spray, similar to a hole in a garden hose. Arterial bloodstains are usually a brighter red due to the chemistry of the blood in the arteries. Expirated blood is that comes from the respiratory system. It can be blown from nose or mouth or be the result of a wound to the system due to air flow and pressure. Such as coughing, sneezing or spitting of blood.


Altered bloodstains are subcategorized as clotted, diluted, diffused, dried, insects, sequenced, and voids. Clotted bloodstains occur as after the bloodletting has occurred the body starts to clot the blood between 3 – 15 minutes in a healthy person. Clotting is a complex mechanism that produces a separation from the liquid portion with a mass that thickens as seen in Photo 7.


Photo 7: a clotted blood stain on a flat surface


Diluted bloodstains ca occur when the blood mixes with any other substance, including other bodily fluids, such as urine or saliva. It also mix with substance in the environment such as cleaners or water, a dilution with water is shown in Photo 8. Diffused bloodstains usually occur on fabric materials as the blood moves from a concentrated area to an area with less or none present. This results in lighter to faint stains around darker staining, usually a saturation stain.


Photo 8: a diluted bloodstain; a blood-water mixture, dropped onto a flat surface, with the center stain also showing a drip pattern with satellite spatter (complex pattern)


Dried or aged stains occur with time, the color changes to a darker red, and a reddish brown and sometimes black. The time for blood to dry depends on the size of the stain and the volume of blood present.


Photo 9: dried bloodstains; Upper is a swipe transfer stain to the right. Lower is a single blood drop. Note the upper stain dried quicker as blood is slightly less and over a slightly larger area


Insects patterns occur when there are flies or other insect activity at the scene as flies can ingest blood and regurgitate it to return later to eat a portion of it. This produces small staining which can be mistaken with impact spatter, so it is important to note if insects are present, as they will remain present as long as the food source remains.

Sequenced stains is determining the order of events that produced changes to the stain patters, this goes to the reconstruction of events and movements through the crime scene event. This includes wipe patterns and skeletonized patterns. A wipe is the altering of a stain caused by movement through the existing bloodstain and could occur through a fresh or partially dried blood stain (Photo 10). A skeletonized stain is a dried stain that may have flakes or have been wiped through. Sequencing can also determine if wipes occurred through transfer stains, or overlapping patterns such as spatter on top of transfer or contact stains.


Photo 10: a partially dried blood stain, a drip pattern stain with satellite spatter, has been wiped through creating a wipe pattern to the right; (complex pattern)


A void is the absence of blood in an otherwise continuous pattern. A void suggests the presence of an intermediate object that may have moved, been moved or removed in its entirety. So if a person moved after blood was shed or in attempt to clean the scene objects were moved or taken away.


Photo 11: A) blood on an item (glove) and surrounding surface B) the same area as photo A, with item (glove) removed resulting in a void pattern, as in this example it can be noted that not all void patterns result in an easily identified object, resulting in investigators attempting to locate an item with a corresponding pattern





The significance of being able to determine and differentiate between the types of patterns is that it can allow the investigator to discover the motions and movement throughout the crime scene. It can also be important to determine the sequence of events. For instance, was the blood present prior to the movement, or is the blood present as a result of movement, or did it just drop. It can also help to potentially determine what caused the motion that altered the stain. Once you are at a crime scene how do you know what stains are blood and thus what to include for your pattern analysis. The crime scene technicians need to perform tests to determine what stains may be blood. Once the patterns are established; all the patterns and the scene as can be reviewed to determine which if samples should be taken and submitted for DNA analysis. There are factors that may also prevent a conclusive interpretation of the bloodstains and the patterns created, this includes instances where minimal staining is present, highly irregular appearance of patterns, surface texture, or lack of information (i.e. limited photos, historical case). In these cases with no conclusive patterns, it may just rely on experience to determine the number and which stains to submit for DNA analysis.

At the crime scenes where violent events have occurred an invaluable tool to the investigators can be the interpretations of the bloodstain pattern analysis.

Disclaimer: This article is for informational purposes only; Maxxam Analytics does not conduct bloodstain pattern analysis.


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Photos 1-3 and 6-11: K. Trudeau, V. Oliver & S. Bax
Photo 4 and 5: V. Oliver