News September 05, 2024

By Forensic Access

Blood Pattern Analysis (BPA) is a cornerstone of forensic science, providing vital insights into the dynamics of violent crimes. Clare Jarman, a distinguished forensic biologist with over two decades of experience, delves into the methodologies, applications, and challenges of BPA in criminal investigations in our recent webinar. This comprehensive overview synthesises key points from that webinar, detailing both the strengths and limitations of BPA in forensic investigations.

Introduction to Blood Pattern Analysis

Blood Pattern Analysis involves the meticulous examination of the shapes, sizes, and distribution of bloodstains in an attempt to reconstruct the sequence of events that occurred as and after blood was shed. BPA can offer crucial information about the positions and movements of victims and perpetrators, the type of weapon used, and a minimum number of blows or shots delivered. By understanding the behaviour of blood under different conditions, forensic scientists can piece together the events of a crime with remarkable accuracy.

Detection and Enhancement Techniques

The initial step in BPA is locating blood at a crime scene. This often begins with a thorough visual examination, utilising various light sources to detect the reddish-brown colour typical of bloodstains. In the laboratory, light sources and low-power microscopes can be employed to identify even minute blood stains on items such as clothing and weapons.

Chemical Testing for Blood: Once potential bloodstains are identified, chemical tests can be used to confirm their presence. The Kastle-Meyer (KM) test is commonly used, involving the application of a colourless chemical that turns pink in the presence of haemoglobin. This test is highly sensitive, capable of detecting blood even when diluted to one in 10,000 parts. Despite its sensitivity, the KM test can occasionally yield false positives with substances like rust or horseradish, necessitating cautious interpretation.  If a stain has the visual appearance of blood and reacts positively to the KM test it can be assumed to be blood.

Luminol and Leuco Crystal Violet: For non-visible bloodstains, luminol is a valuable reagent. It reacts with the haemoglobin in blood to produce, instead of a colour change, a chemiluminescent glow, revealing hidden blood patterns in darkened environments. This allows the visualisation of blood patterns on any surface regardless of background colour. Luminol is exceptionally sensitive, detecting blood diluted up to one in a million and as such can be highly useful in showing evidence of clean up at a scene, or when searching for very old or dilute blood.  However, its lack of a fixative can readily cause blood to run, distorting patterns, particularly on non-porous surfaces. Luminol can also react with other substances such as vegetation, bleach and metals and as such it can only be used to indicate that blood may be present, rather than confirm its presence.  In contrast, Leuco Crystal Violet (LCV), which changes colour to purple in the presence of haemoglobin, includes a fixative, preserving the integrity of blood patterns, LCV is highly sensitive and, similarly to KM displays far fewer false positive reactions.     However, LCV brings with it some health and safety concerns which makes it more suited to application in a laboratory setting

Types of Blood Patterns

Bloodstains are generally categorised into two primary types: contact stains which occur when a surface wet with blood comes into contact with another surface and airborne stains resulting from blood droplets travelling through the air and landing on a surface.

Contact stains can include:

  • Transfer patterns such as a finger mark, footwear mark or fabric mark in blood
  • Swipes where a bloodstained surface moves across another surface
  • Wipes where wet blood already on a surface is wiped across that surface

Airborne type patterns can include:

Drip Stains: Formed by blood falling under the influence of gravity, drip stains can indicate the movement of a bleeding individual or item wet with blood. The size and shape of these stains are influenced by the volume of blood, the height from which it falls, and the surface it contacts. Drip stains on non-porous surfaces like laminate flooring spread out forming larger circular stains, while those on porous surfaces like carpet appear smaller due to absorption.

Impact Spatter: Created when an external force, such as a blow, causes wet blood to break into droplets and travel through the air forming spots on the surrounding surfaces on which they land. The shape and distribution of the resultant spots can help identify where the source of the wet blood was when struck and potentially a minimum number of blows delivered on that wet blood.

Cast-Off Patterns: These occur when blood is flung from a blood-bearing object in motion, such as a weapon or hand. Cast-off patterns typically form a linear pattern of stains, that can assist in addressing where the item was when it moved.

Gunshot Spatter: This typically appears as a fine mist of blood droplets. This type of spatter can help identify the positions of the shooter and the victim. Gunshot spatter can include forward spatter, produced as the projectile exits the body, and back spatter, directed back towards the shooter as the projectile enters.

Expirated Blood: Blood expelled from the mouth, nose, or a wound to the lungs/airways, often mixed with saliva, creating a pattern that can be mistaken for impact spatter. Characteristics like air bubbles within the stains can help differentiate expirated blood from impact spatter, however in some instances it will not be possible to determine whether the pattern is the result of impact or expiration.

Arterial Patterns: When an artery is breached, blood is pumped out under high pressure, creating distinctive patterns known as arterial gushes and spurts. These patterns are produced with each heartbeat, forming characteristic clusters of stains that can indicate the location and movement of the individual after the artery was severed.

The Process of Blood Drying

 The drying time of blood is influenced by numerous factors, including temperature, humidity, airflow, and the volume of the bloodstain.

Factors Affecting Drying Time:

  1. Temperature: Higher temperatures accelerate the drying process, while lower temperatures slow it down. Blood on a hot, sunny surface can dry within seconds, whereas in a cooler environment, it may take much longer.
  2. Humidity: High humidity levels can prolong drying times, as the moisture in the air slows the evaporation process. Conversely, low humidity speeds up drying.
  3. Airflow: Increased airflow across a bloodstain facilitates faster drying by promoting evaporation. Stagnant air slows the process significantly.
  4. Volume of Blood: Larger volumes of blood take longer to dry than smaller volumes. A small blood droplet on a non-porous surface might dry in a few minutes, while a larger pool can take hours or even days.
  5. Surface Type: The surface on which the blood lands also affect drying times. Non-porous surfaces like glass or metal allow blood to dry faster due to minimal absorption, while porous surfaces like fabric or carpet absorb blood, potentially extending the drying process.

Practical Implications:

Accurately estimating the drying time of blood remains challenging due to the multitude of influencing factors. Environmental conditions at the crime scene, such as fluctuating temperatures or varying humidity levels, can complicate assessments. Additionally, bloodstains on different materials or in various locations within the same scene might dry at different rates.  Therefore, it is generally not possible to give anything more than approximate time frames at best.

The Role of DNA in Blood Pattern Analysis

DNA analysis is a critical component of forensic investigations, particularly in conjunction with BPA. Identifying the potential source of bloodstains through DNA profiling allows forensic scientists to link blood evidence to specific individuals, thereby strengthening the reconstruction of crime scenes.

DNA Profiling of Bloodstains:

Once blood is detected and its patterns are analysed, DNA profiling can be performed to determine the blood's potential origin. Forensic scientists can extract DNA from even minute quantities of blood, obtaining a profile that can be compared to known reference samples.

Human vs. Animal Blood:

Chemical tests like KM and luminol do not distinguish between human and animal blood. However, the DNA profiling techniques used are specific to areas of human DNA and so if a DNA profile is obtained, this indicates the blood is human in origin. Therefore, DNA analysis is essential to confirm the human origin of bloodstains. 

Challenges in DNA Analysis:

The effectiveness of DNA analysis depends on the quality and quantity of the blood sample. Non-visible or very light bloodstains may yield weak DNA profiles, making it difficult to draw definitive conclusions. Additionally, environmental factors, such as exposure to heat or moisture, can degrade DNA, affecting the reliability of the results.

Experimentation and Training

Experimentation and training are crucial for enhancing the accuracy and reliability of a scientist’s conclusions with regard to BPA. Forensic scientists often conduct controlled experiments using animal blood to simulate human blood patterns. These experiments help develop a deeper understanding of how blood behaves under various conditions and inform the interpretation of bloodstains at actual crime scenes and on items.

Mock crime scenes are set up to train forensic scientists, allowing them to practice identifying and interpreting blood patterns. These simulations provide valuable hands-on experience and help scientists refine their skills in real-world applications. At Forensic Access, we offer specialized courses designed to enhance the skills of forensic practitioners in BPA.

Blood Pattern Analysis Awareness for Clothing: This course focuses on the interpretation of blood patterns on clothing, providing participants with an awareness of the knowledge and practical skills needed to accurately analyse and interpret bloodstains on garments and an overview of the associated strengths and limitations.

Blood Pattern Analysis Awareness for Scenes: This course is tailored to understanding blood patterns at crime scenes. It provides participants with an awareness of the types of blood patterns often encountered at scenes and how this information can be used to assess what events may have occurred.  It also provides an overview if the strengths and limitations of blood pattern evidence. These courses are integral to the continuous professional development of forensic practitioners, ensuring they are equipped with the latest techniques and knowledge in blood pattern analysis.

For more information on these courses, visit our BPA Awareness for Clothing and BPA Awareness for Scenes pages.

Case Studies and Real-Life Applications

Real-life case studies illustrate the practical applications of BPA. For example, the examination of blood patterns on clothing can provide insights into the actions and positions of individuals during an assault. One particularly revealing case involved the analysis of bloodstains on a pair of trainers.

The Trainer Case Study:

In this case, a pair of trainers was submitted for examination after an alleged assault. The complainant stated that he had been punched repeatedly to the head and had then fallen to the ground. The defendant stated he had struck the complainant to the head but in self-defence. The prosecution initially interpreted the blood patterns on the trainers of the defendant as supportive of kicking into wet blood. The bloodstains, as reported by the prosecution, included a pooling of blood within the welt towards the front of the toe area, contact bloodstains on the toe, and numerous small spots of blood. The subsequent interpretation suggested that the wearer of the trainers had kicked the already bleeding complainant, significantly impacting the charges and the prosecution's narrative.

However, upon re-examination by our biology team, the interpretation of the bloodstains was questioned. The defence team conducted a series of experiments to simulate different scenarios involving blood dripping down onto trainers and into adjacent pools of blood on the floor. The findings showed that highly similar bloodstain patterns could be produced by blood dripping in this way with no contact between the trainers and the bleeding person. The re-evaluation led to a reconsideration of the charges. This case underscores the importance of robust, peer reviewed analysis and the potential subjectivity within this area. It highlights the need for forensic scientists to consider multiple scenarios and conduct experiments to validate their interpretations. The case also demonstrates the importance of BPA within the criminal justice system and the very real dangers associated with misinterpretation and over interpretation of any blood patterns present. By integrating detailed case studies such as this, forensic professionals can enhance their understanding of BPA and its applications in real-world scenarios, ensuring that forensic evidence is interpreted accurately and justly.

Conclusion

Blood Pattern Analysis is an indispensable tool in forensic investigations, offering detailed insights into the events surrounding violent crimes. However, BPA must be approached with caution, a robust knowledge of the strengths and limitations of the discipline and a solid grounding in the various factors that can influence blood patterns. Continued research, experimentation, and training are essential to enhancing the scientist’s effectiveness and ensuring accurate, reliable forensic analysis.

Contact Forensic Access

If you require an expert report involving any aspect of Blood Pattern Analysis, or any other forensic discipline please contact our Casework Management Team by email: science@forensic-access.co.uk or call: 01235 774870.