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Bloodstain Patterns—A Primer for Lawyers

What is Bloodstain Pattern Analysis? 

Bloodstain pattern analysis (“BPA”) is the scientific examination and interpretation of bloodstains at crime scenes. It establishes events which could or could not have occurred during and after bloodshed. BPA involves two stages: 
1. Pattern interpretation examines, evaluates and categorises each pattern based on its physical characteristics (eg, distribution, shape and size of the bloodstains), its relative location and relationship with other bloodstains at the scene, as well as the nature of the target surface where the stains were deposited.
2. Reconstruction of events provides contextual explanations to the bloodstain patterns by postulating possible human actions and likely activities at the scene. 
The interpretation of bloodstain patterns has been documented in books, journals and articles for more than 100 years. Recognition of BPA as a forensic discipline in the modern era dates back to 1955 when the renowned criminalist Dr Paul Leland Kirk applied BPA in the well-known case of State of Ohio v Samuel H. Sheppard, providing critical evidence that eventually led to Sheppard’s acquittal in a retrial more than 12 years after the murder. This was a significant milestone on the acceptance of bloodstain evidence by the American legal system.
What is the mechanism behind this pattern?

How Much Should You Know About BPA? 

This question is best answered by Professor Andre A. Moenssens in his 1993 article Novel Scientific Evidence in Criminal Cases: “If the attorneys who are questioning and cross-examining the expert witnesses have a working knowledge of BPA, they will be better equipped to distinguish between those experts with sufficient qualifications and those without them. Also, attorneys will be able to more capably critique an expert’s testimony and limit or eliminate conclusions that are speculative or over-stated. Unless attorneys know what questions to ask during cross-examination, much of what the witness testifies to will go unchallenged”.

Principles of BPA 

The behaviour of blood is based on widely accepted and well-established scientific principles of mathematics, physics and biology.1 Not only does blood shed in a predictable manner, blood that is shed reacts to external forces in a predictable manner. Therefore, bloodstain patterns are broadly predictable and reproducible under similar conditions. To interpret bloodstain patterns, the following must be understood:
Properties and behaviour of blood: Blood is a colloidal fluid, consisting of a fluid portion known as plasma, and solids comprising red blood cells, white blood cells and platelets. In healthy adults, the total blood volume ranges from 4.5 to 6.0 litres. Blood is in a liquid state inside the body, but on exiting the body, it starts to clot within three to 15 minutes to form a darker gelatinous substance. The appearance and extent of clotted blood at a scene may indicate how much time has elapsed after the bloodletting injury occurred. 
Formation of patterns: The characteristics of a pattern depend on factors such as:
Adhesive properties (attractive forces between blood and a surface) and cohesive properties (between blood molecules) of blood;
Type of injury inflicted, the blood vessel breached, and the speed at which blood exits the wound;
Type and amount of force applied to the blood source; 
Volume of blood shed;
Angle at which the blood drop struck the surface (angle of impact);
Distance travelled by the blood drop; and
Texture and porosity of the surface the blood lands on.    
Area of origin: The angles of impact are calculated from bloodstain dimensions and trigonometry. Further calculations indicate the location where a blood shedding object was impacted. 
Angle of Impact – Bloodstains resulting from blood drops striking cardboard surfaces at angles of 10°, 30°, 70° and 90°

Types of Bloodstain Patterns and Where They are Found

Bloodstain patterns are commonly associated with violent crimes such as assaults, homicides and suicides, and with traffic collisions. They are found on surfaces of walls, furniture and objects, and on the clothing and bodies of persons present at the scene. The amount of bloodstain evidence present depends on the circumstances of the crime: the type and intensity of the activities, the type of bleeding wounds, and whether clean-up was carried out after bloodshed. 
Bloodstain pattern examiners typically adopt the terminology (see Table 1) recommended by the Scientific Working Group on BPA (SWGSTAIN).2 Bloodstain patterns are classified into three categories: passive, transfer and spatter patterns.
1. Passive patterns include drip stains, drip trails, drip patterns, flow patterns and blood pools. As the name implies, no external force except gravity is involved in their formation. 
2. Transfer patterns comprise wipes, swipes and pattern transfers such as a bloody shoeprint, footprint or palm print or an imprint of a tool or object stained with wet blood. These patterns are formed when a blood-bearing surface comes into contact with another surface.   
3. Spatter patterns include cast-offs, splash, expiration, projected and impact patterns. They result from blood drops projecting and dispersing through the air due to an external force applied to a source of liquid blood. The characteristics of spatter stains depend on the speed at which the blood exits the wound and the force applied to a blood source. Spatter patterns comprise small and tiny bloodstains which are typically smaller than passive stains. 
Transfer Patterns – Hair Swipe (top), Wipe (bottom right), Palm Print (bottom left)
Table 1: Glossary of terms for bloodstain evidence

Evidential Value of BPA

Blood is “sticky”; not only does it adhere to surfaces, it is not easy to totally clean and remove bloodstains, leaving no trace of them. Unlike trace evidence which is transient, under the right conditions, dried bloodstains can remain intact and preserved for many years. Bloodstain patterns are invaluable for identifying persons involved in the bloodshed, establishing possible criminal events, and providing crucial investigative leads. The evidential value of bloodstain patterns is illustrated below:
1. Drip trails indicate movement between two locations; the shape of the drip stains in the trail indicates directionality and whether the injured person was moving slowly or running. 
2. Flow patterns on vertical surfaces indicate either contact with a significant amount of blood, or prolonged contact with a bleeding wound. If a surface with a flow pattern is re-oriented, the altered pattern will reveal the initial orientation of the object when the blood was flowing.
3. Imprints of tools at the scene and on fabric material can be traced to the weapon (or weapon type) used to stab or hit a person.
4. Cast-off patterns can identify the type of weapon, and the direction and minimum number of blows delivered. 
5. Impact spatter patterns may pinpoint the area of origin of the blood source. If found on the suspect’s clothing or body, it may place the person at the crime scene. 
6. Void patterns are useful for establishing the position of a person at the scene. 
Correlating information: BPA complements findings from the DNA profiling, medical and autopsy reports. The DNA profiles of bloodstains, medical findings on injuries present on the victim and suspect, and autopsy findings on injuries on the deceased should be correlated with bloodstain patterns found at the scene and on clothing in order to ensure consistency of findings. For example, in the case of PP v Tan Chor Jin,3 the forensic pathologist reported that the deceased Lim Hock Soon’s left lung had collapsed as a result of the gunshot at the back left shoulder, and his airways contained frothy blood. This injury corroborated the expiration pattern found on the floor of the room where six shots had been fired at the deceased. 
The 4Ws and H of a crime: At the scene, bloodstains are judiciously selected and collected from different bloodstain patterns for confirmation of human blood and development of DNA profiles. While the DNA profiles identify the “Who” in the incident, BPA corroborates, refutes or clarifies the statements of the eye witness, victim and suspect by independently uncovering critical information on the “What”, “When”, “Where” and “How” of events and actions:4 
What occurred and what did not occur?
Where did the events occur?  
When did they occur and in what sequence?
Who were present?
Where were these persons relative to each other and to the scene during bloodshed?
Answers to these interrogatives are particularly useful in cases where a suspect can be linked to the incident but information on actions, individual responsibilities, and the role of other persons present at the scene is incomplete, unclear or disputed. To test hypotheses, the scientist may conduct simulation experiments using specific case parameters, variables and mechanisms to recreate bloodstain patterns. 
Simulation experiment – multiple impacts on a blood source using a hammer. Take note of the spatter stains on the tool, and on the sleeves and gloves of the person who impacted the blood source.

Limitations of BPA 

Bloodstain patterns tell with reasonable certainty what likely occurred at specific moments in time. However, it is unable to produce a reconstruction of ALL the events that occurred during the incident and their sequence because of gaps in information and limitations of this discipline. Assumptions and qualifiers to address the limitations must be clearly stated in bloodstain pattern reports. Common limitations are discussed below:
Several factors render a bloodstain pattern unusable:
Alterations of bloodstains by first responders and paramedics;
Alterations of bloodstains on the clothing of the deceased during transport to the mortuary;
Washing away of bloodstains due to rain or clean-up by the victim, suspect or other persons with access to the scene; 
Obliteration of a useful pattern by another overlapping pattern, making it unrecognisable; and 
Lack of bloodstains as a result of minimal number of blood-letting wounds or insignificant wounds which clot rapidly.
In the case of PP v Aguilar Guen,5 the suspect took a few days to dismember the deceased, pack the body parts, dispose of them and clean up the scene. A significant amount of bloodstains on the wall in Guen’s bedroom had been wiped away, leaving only a few spatter stains. Although luminol was able to chemically enhance the invisible traces of blood to reveal them sufficiently for photo-documentation, it provided only gross information such as the presence of transfer patterns, it could not reveal the small spatter stains which had been wiped away. 
Absence of evidence: Conclusions in BPA should be based on bloodstains that were physically present, and not on bloodstains the examiner expects to be present, unless there is strong circumstantial evidence to suggest otherwise. For illustration, a suspect described in his statement his position at the location where bloodshed occurred. If an impact spatter pattern containing a void is found at the location described by the suspect, one would expect to find spatter stains on him corresponding to the void. 
Reliance on photographic evidence only: Limitations are encountered in interpretations based solely on photographic evidence. When interpreting bloodstain patterns from a remote scene (ie an “old” scene), the examiner has to rely on scene photographs, as the original scene has been cleared. Photographs are two-dimensional, highly dependent on perspective and zoom, and often poorly represent spatial relationships between objects at the scene. Selection by the photographer comes into play: a lack of overview photographs depicting the location of a bloodstain pattern at the scene will cause a pattern to lose its context.6 If too few close-up views were taken, details in a pattern and small stains may not be discernible. The area of origin where impact likely occurred can be determined only if stain sizes were measured and their relative locations documented. Hence, this indirect examination through photographs may suffer from discontinuities and gaps in information, and missed evidence. Details and answers sought by the scientist may no longer be available. In a nutshell, the comprehensiveness of the interpretation depends significantly on the adequacy of the photo-documentation, the type of bloodstain patterns present at the scene, and the feasibility of a visit to the remote scene to appraise spatial relationships.     
Oversimplification: A major pitfall is oversimplification of this complex discipline. Oversimplification occurs when the examiner fails to consider all the available evidence, facts and factors, and does not fully apply rigorous scientific methods. 
Different spatter mechanisms may produce spatter patterns which may appear strikingly similar, for example, expirated bloodstains versus impact spatter stains due to beating. For such cases, it may be difficult to distinguish between plausible mechanisms, unless additional information allows more specific interpretation and identification of the mechanism. In the above example, the presence of saliva and air bubbles in the pattern would indicate expirated blood. A prudent examiner would carefully consider all the possible mechanisms for a pattern, ensure that the mechanisms corroborate medical, autopsy and DNA profile findings, and highlight in the expert report and testimony any instances where there could be more than one plausible explanation for a bloodstain pattern.  
Expertise and skills: To avoid erroneous attribution to a single cause, proficiency in BPA is imperative. The examiner must possess a strong foundation in the scientific method, master the science and skills associated with BPA, and understand its limitations. Practical experience across a wide range of cases and evidence types is essential. While interpreting evidence, the examiner must constantly articulate, challenge and refine arguments using logic and sound reasoning. The BPA expert must always be open-minded and objective in considering and reporting the full array of possibilities and explanations for a bloodstain pattern. 

Local Cases Involving BPA

In Singapore, the value of BPA became more evident after this forensic discipline was successfully used to reconstruct blood shedding events for several major cases in 2005. The significance of bloodstain patterns in providing “the critical link” and its limitations in the reconstruction of a crime is illustrated through three of our past cases. 
Yishun Triple Murder (PP v Wang Zhijian)7 
Wang Zhijian, a 45-year old man was charged with the murder of three women, Zhang Meng, Feng Jianyu and Yang Jie in a flat in September 2008. Yang, who was clinging to laundry pole holders outside the kitchen windows fell to her death. Wang admitted to causing the deaths of Zhang and Feng, but denied any involvement in Yang’s death. He also disputed the sequence of events after the killing of Zhang and Feng. The only witness was Li Meilin, Feng’s daughter who survived despite being attacked by Wang. 
Transfer patterns in the living room, bedrooms and kitchen helped establish the sequence of events and movements of the five persons. The deep and neat cut across Yang’s fingers, and the presence of her bloodstains only on the exterior surface of the kitchen wall near the laundry pole holders but nowhere else in the unit suggested that Yang’s fingers were cut by a sharp knife while she was clinging outside the kitchen window.  
The Judge accepted the BPA findings and crime scene reconstruction reports, and relied on the expert’s testimony in sentencing Wang. He found that Li’s evidence “on the whole was consistent with the evidence given by” the expert and indicated that “her evidence was independently corroborated by the forensic evidence and major points in the Reconstruction Report”. The Judge further accepted “the forensic analysis” and “the conclusion … reached in the Reconstruction Report based on the vast amount of forensic evidence gathered from the scene of crime”. 
Suicide of NTU student (CI of David Hartanto Widjaja)8 
On 2 March 2009, 21-year-old Indonesian David Hartanto Widjaja, a final-year NTU School of Electrical and Electronic Engineering student met his project supervisor, A/Professor Chan Kap Luk in the latter’s office to discuss his project. Moments later, loud screams were heard, and Chan staggered out of his office with a stab wound in his back, holding a broken knife blade in his hand. David ran out of the office barefooted, and subsequently fell a few storeys to his death from the glass roof of a link bridge. Police suspected that David had attacked Chan and subsequently committed suicide. However, David’s family alleged that he was murdered. The case quickly escalated into a politically sensitive issue involving the Indonesian media.  
Forensic scientists were activated about a month after the incident to reconstruct this remote scene.  Relying on scene photographs taken previously by police officers, visits were made to the scene to examine the spatial relationships between locations and objects, and correlate the scene with the photographs depicting the bloodstains. The scene in Chan’s office had been preserved but the bloodstains in the corridors and stairways had been cleaned up. Despite the clean-up, small spatter stains and transfer stains were found on the staircase railings and parapet wall leading to the link bridge.    
Scene photographs depicted an undisturbed long drip trail left behind by David, stretching from the office corridor, down two flights of stairs, across the lift lobbies, over the parapet wall and onto the glass roof of the link bridge. This unaltered pattern and the transfer pattern on the parapet wall indicated that David was not accompanied by other persons. Close-up photographs of the void pattern on the link bridge, the saturation stain at the back of David’s trousers and his significantly stained feet provided strong evidence that blood was dripping actively from David’s wounds onto his feet and he sat on the bloodstained glass roof before falling to his death.  
The reconstruction of events based on bloodstain pattern evidence and simulation on how the knife blade fractured were presented at the Coroner’s Inquiry. The State Coroner passed a verdict of suicide. 
Cell-phone heist and murder (PP v Daniel Vijay s/o Katherasan and another)9 
Based on insider information, three men staged a road accident to stop a lorry carrying $1.3 million worth of cell-phones. While committing the robbery, they assaulted the driver, Wan Cheon Kem who died six days later from severe head injuries. Glass fragments on the roadside along Changi Coast Road matched glass from the broken window on the left side of the lorry cabin, indicating the site where the lorry stopped, and where Wan was attacked. The dented area with spatter stains in and around the dent, located at a significant height on the metal door frame suggested that a long hard object had been used in the attack while the cabin door was open. Impact spatters inside the cabin indicated that the driver could have been struck again in the cabin. While BPA was useful in establishing what happened to Wan, where and how the attacks took place, and the type of weapon used, it could not provide information to indicate who among the suspects struck him with the weapon as the clothing of the suspects and the weapon were not available for BPA. 


BPA provides the trier of fact with invaluable associative evidence. It provides answers to the “What”, “Who”, “When”, “Where” and “How” of a crime, and is essential for scene reconstruction10 of violent crimes. It establishes linkages in an incident, and uncovers details of individual responsibilities. Like all other physical evidence, bloodstain patterns should not be considered in isolation, but should be interpreted with other evidence types. BPA is often synergistic in probative value when combined with trace evidence,10 marks, prints and damage,10 and other physical evidence. The whole is often greater than the sum of the individual parts.

What’s Next?

Look out for the next article “Fires and Explosions”. The article addresses the science behind the discipline, the methodology and challenges encountered in examining and reporting on cases involving fires and explosions.  

The Forensic Experts Group*
E-mail: [email protected]
* The Forensic Experts Group (“TFEG”) is Singapore’s first one-stop private and independent provider of forensic consultancy, analysis, research, training and education for legal and law enforcement agencies, forensic and tertiary institutions, and private organisations. It comprises a team of accomplished forensic scientists, who are combining 75 years of specialised knowledge, unique experience and skillsets to deliver high quality forensic services both locally and overseas. While heading the Forensic Chemistry & Physics Laboratory at the Health Sciences Authority, TFEG’s senior consultants developed and instituted bloodstain pattern analysis as a distinctive forensic discipline in Singapore, successfully applying the expertise to many high profile local cases since 2005. 


1 Stuart H. James, Paul E. Kish, T. Paulette Sutton, Principles of Bloodstain Pattern Analysis: Theory and Practice (CRC Press). 
2 Scientific Working Group on Bloodstain Pattern Analysis: Recommended Terminology. 
3 PP v Tan Chor Jin [2007] SGHC 77; [2008] 4 SLR 306; [2008] SGCA 32.
4 Tom Bevel, Ross M. Gardner, Bloodstain Pattern Analysis: With an Introduction to Crime Scene Reconstruction (third edition, CRC Press).
5 PP v Aguilar Guen [2006] 3 SLR 247; [2006] SGHC 94. 
6 Supra, (note 4 above).
7 PP v Wang Zhijian [2012] SGHC 238; PP v Wang Zhijian and Another appeal [2014] SGCA 58.
8 Scene reconstruction: David Widjaja’s last moments, The Online Citizen (27 May 2009).
9 PP v Daniel Vijay s/o Katherasan and Others [2008] SGHC 120; [2010] SGCA 33. 
10 The Forensic Experts Group, “The Essence of Forensic Science”, Singapore Law Gazette (January issue 2015); “Forensic Reconstruction”, Singapore Law Gazette (May issue, 2015); “Trace Evidence” Singapore Law Gazette (July issue, 2015); “Damage, Marks and Prints” Singapore Law Gazette (October issue, 2015).