How important is it to consider target properties and hematocrit in bloodstain pattern analysis?

Alginate/xanthan gum hydrogels as forensic blood substitutes for bloodstain formation and analysis

Understanding the behaviour of human blood outside of the body has important implications in forensic research, especially related to bloodstain pattern analysis (BPA). The design of forensic blood substitutes (FBSs) can provide many advantages, including the incorporation of multiple physiological components for use as safe and reliable materials for forensic applications. In this work, we present the design of synthetic alginate and xanthan gum-based hydrogels that contain electrosprayed microparticles (MPs) with and without crosslinked DNA. In addition to the MPs, the alginate/xanthan gum FBS materials include fillers to alter the physical appearance and fluid properties of the material. The optimized FBS consisted of alginate (1% w/v) and xanthan gum (5.0 × 10^-3% w/v), 2 mM CaCl₂, ferric citrate (0.5% w/v), magnesium silicate (0.25% w/v), Allura Red dye (2% w/v), 0.025% v/v Tween 20 and 9.5% v/v MPs. The FBS was tested in passive dripping experiments relevant to BPA scenarios at various impact angles. The spreading ratio (D_s/D₀) was determined for 90° stains made on a paper surface and compared to bovine blood where the FBS was shown to simulate accurate and predictable spreading behaviour. In addition, we simulated other common BPA scenarios (e.g., impact patterns) and evidence processing potential. The FBS could be swabbed, and the DNA could be extracted, amplified, and genotyped analogous to human blood evidence. A stability test was also conducted which revealed a shelf-life of over 4 weeks where the material remains relevant to human blood at physiological temperature.

Using the likelihood ratio in bloodstain pattern analysis

There is an apparent paradox that the likelihood ratio (LR) approach is an appropriate measure of the weight of evidence when forensic findings have to be evaluated in court, while it is typically not used by bloodstain pattern analysis (BPA) experts. This commentary evaluates how the scope and methods of BPA relate to several types of evaluative propositions and methods to which LRs are applicable. As a result of this evaluation, we show how specificities in scope (BPA being about activities rather than source identification), gaps in the underlying science base, and the reliance on a wide range of methods render the use of LRs in BPA more complex than in some other forensic disciplines. Three directions are identified for BPA research and training, which would facilitate and widen the use of LRs: research in the underlying physics; the development of a culture of data sharing; and the development of training material on the required statistical background. An example of how recent fluid dynamics research in BPA can lead to the use of LR is provided. We conclude that an LR framework is fully applicable to BPA, provided methodic efforts and significant developments occur along the three outlined directions.

Animal blood in translational research: How to adjust animal blood viscosity to the human standard

Animal blood is used in mock circulations or in forensic bloodstain pattern analysis. Blood viscosity is important in these settings as it determines the driving pressure through biomedical devices and the shape of the bloodstain. However, animal blood can never exactly mimic human blood due to erythrocyte properties differing among species. This results in the species-specific shear thinning behavior of blood suspensions, and it is therefore not enough to adjust the hematocrit of an animal blood sample to mimic the behavior of human blood over the entire range of shear rates that are present in the body. In order to optimize experiments that require animal blood, we need models to adapt the blood samples. We here offer mathematical models derived for each species using a multi linear regression approach to describe the influence of shear rate, hematocrit, and temperature on blood viscosity. Results show that pig blood cannot be recommended for experiments at low flow conditions (<200 s-1 ) even though erythrocyte properties are similar in pigs and humans. However, pig blood mimics human blood excellently at high flow condition. Horse blood is unsuitable as experimental model in this regard. For several studied conditions, sheep blood was the closest match to human blood viscosity among the tested species.

Nano-Interstice Driven Powerless Blood Plasma Extraction in a Membrane Filter Integrated Microfluidic Device

Blood plasma is a source of biomarkers in blood and a simple, fast, and easy extraction method is highly required for point-of-care testing (POCT) applications. This paper proposes a membrane filter integrated microfluidic device to extract blood plasma from whole blood, without any external instrumentation. A commercially available membrane filter was integrated with a newly designed dual-cover microfluidic device to avoid leakage of the extracted plasma and remaining blood cells. Nano-interstices installed on both sides of the microfluidic channels actively draw the extracted plasma from the membrane. The developed device successfully supplied 20 μL of extracted plasma with a high extraction yield (~45%) in 16 min.

Bloodstain pattern analysis: Does experience equate to expertise?

Bloodstain pattern analysis (BPA) has long been accepted by courts as an area of expertise; however, that position has recently been challenged. The discipline has been criticized for limited empirical research into practitioner determination error rates and whether determinations require specialized knowledge/expertise, including whether practitioner experience level influences accuracy. This study attempted to address these knowledge gaps as they relate to bloodstain pattern recognition. The aims were twofold: to establish whether practitioners would outperform lay non-practitioners, and whether practitioner experience influenced accuracy and error in determinations. Comparisons of practitioner responses under three scenarios (forced, casework, and definitive) were also made to assess conservatism/certainty in pattern recognition. Participants (both BPA practitioners and non-practitioners) analyzed photographs of bloodstain patterns and made determinations of the broad bloodstain category and specific bloodstain pattern type. When forced to provide only a single response, practitioners identified bloodstain categories and patterns significantly more accurately than non-practitioners (p = 0.0001, p < 0.00001, respectively). Practitioner accuracy in bloodstain pattern recognition was positively associated with experience level (p = 0.0429) and this was consistent regardless of response scenario. Although no significant difference in practitioner accuracy was observed across response scenarios, practitioner conservatism/certainty varied significantly among the broad bloodstain category and specific pattern types. Overall, these results support bloodstain pattern recognition as an area of expertise and that practitioner experience positively influences accuracy. Based on these results, a series of recommendations were proposed aimed at further improving practices within the discipline to maximize accuracy and reliability of BPA evidence.

Charts based on millions of fluid dynamics simulations provide a simple tool to estimate how far from its source a specific blood stain can be found

The bloodstain pattern analyst sometimes has to judge if a given stain could originate from a specific location. A wide range of values of the maximum distance that a blood drop can travel have been reported from experiments, ranging from less than one meter to more than 10 m. Here we formulate the problem in a fluid dynamics and data mining framework. The fluid dynamics is solved with Newton's classical equation of motion coupled with well-established models for the gravity and drag forces that bend the trajectories of drops. The parameters screened are the drop size, initial velocity and launch angle, as well as the height of a blood source and the ceiling height. Combining a wide range of values of those five parameters commended the performance of more than 5 million fluid dynamic simulations. Results of these simulations have been searched and mined for parameters directly measurable on a crime scene, such as the stain size and stain ellipticity. The results are presented in simple, easy to use charts, which do not require any knowledge of fluid dynamics from the analyst.

A data set of bloodstain patterns for teaching and research in bloodstain pattern analysis: Gunshot backspatters

This is a data set of blood spatter patterns scanned at high resolution, generated in controlled experiments. The spatter patterns were generated with a rifle or a handgun with varying ammunition. The resulting atomized blood droplets travelled opposite to the bullet direction, generating a gunshot backspatter on a poster board target sheet. Fresh blood with anticoagulants was used; its hematocrit and temperature were measured. The main parameters of the study were the bullet shape, size and speed, and the distance between the blood source and target sheet. Several other parameters were explored in a less systematic way. This new and original data set is suitable for training or research purposes in the forensic discipline of bloodstain pattern analysis.

Determining the region of origin of blood spatter patterns considering fluid dynamics and statistical uncertainties

Trajectory reconstruction in bloodstain pattern analysis is currently performed by assuming that blood drop trajectories are straight along directions inferred from stain inspection. Recently, several attempts have been made at reconstructing ballistic trajectories backwards, considering the effects of gravity and drag forces. Here, we propose a method to reconstruct the region of origin of impact blood spatter patterns that considers fluid dynamics and statistical uncertainties. The fluid dynamics relies on defining for each stain a range of physically possible trajectories, based on known physics of how drops deform, both in flight and upon slanted impact. Statistical uncertainties are estimated and propagated along the calculations, and a probabilistic approach is used to determine the region of origin as a volume most compatible with the backward trajectories. A publicly available data set of impact spatter patterns on a vertical wall with various impactor velocities and distances to target is used to test the model and evaluate its robustness, precision, and accuracy. Results show that the proposed method allows reconstruction of bloodletting events with distances between the wall and blood source larger than ∼1 m. The uncertainty of the method is determined, and its dependency on the distance between the blood source and the wall is characterized. Causes of error and uncertainty are discussed. The proposed method allows the consideration of stains indicating impact velocities that point downwards, which are typically not used for determining the height of the origin. Based on the proposed method, two practical recommendations on crime scene documentation are drawn.

Determination of the maximum distance blood spatter travels from a vertical impact

Bloodstain evidence can be very powerful evidence in assault related crimes. Determination of the distance that blood droplets may travel as a result of an impact into liquid blood may be of significance to corroborate or disprove a version of events, provide likely scenarios, or help determine the culpability of a person in determining their proximity to the blood shedding event. It was the aim of this research to determine the potential maximum distance blood droplets travel horizontally following a vertical impact into liquid blood. A custom apparatus was designed and constructed to replicate a vertical impact of a timber weapon, rotating on a fixed axis at one end, striking a pool of liquid blood. The device was positioned at three different levels of elevation to replicate an impact to the head of a person near ground level, a seated or kneeling height and standing height. Overall, the results indicated that the application of kinetic energy of between 1 and 5J at a height of 1780mm led to the blood droplets travelling a maximum horizontal distance of 5361mm (and average maximum distance of 4981mm). The horizontal distance blood droplets may travel upon impact does not appear to follow a linear trend with differing kinetic energy, but is affected by the applied force and release height in a curvilinear relationship. The results provide a valuable tool to bloodstain pattern analysts and investigators in determining search zones within a scene, as well as providing information about the proximity of an individual to an impact event.

A data set of bloodstain patterns for teaching and research in bloodstain pattern analysis: Impact beating spatters

This is a data set of 61 blood spatter patterns scanned at high resolution, generated by controlled impact events corresponding to forensic beating situations. The spatter patterns were realized with two test rigs, to vary the geometry and speed of the impact of a solid object on a blood source – a pool of blood. The resulting atomized blood droplets travelled a set distance towards a poster board sheet, creating a blood spatter. Fresh swine blood was used; its hematocrit and temperature were measured. Main parameters of the study were the impact velocity and the distance between blood source and target sheet, and several other parameters were explored in a less systematic way. This new and original data set is suitable for training or research purposes in the forensic discipline of bloodstain pattern analysis.

Effect of anticoagulation therapy on drying times in bloodstain pattern analysis

In forensic case work, blood stain pattern analysis frequently aids in deducing the chain of actions or parts thereof taking place during an event leading to blood loss. Wiped single blood stains and/or groups of blood stains are seen at a majority of complex crime scenes. The appearance of wiped blood stains depends on droplet volume and stain age (as a function of blood viscosity and the degree of stain skeletonization) and characteristics of the stained surface (i.e., texture, temperature). Furthermore, based on the biochemical and biophysical properties of blood, not only the drying processes, but also complex coagulation cascades are relevant to the assessment of wiped blood stains. This study was designed to determine if anticoagulation therapies markedly affect the wipeability of blood stains over times elapsed since deposition and the overall drying process. A total of 813 blood stains, originating from donors being treated with acetylsalicylic acid (ASA), clopidogrel + ASA, low-molecular-weight heparin, or rivaroxaban, were dropped on common household tiles. Wipeability at an ambient temperature of 20 °C was tested for 22 time periods (1, 2, 3, 5, 10, 15…105 min since deposition). Whereas stains consisting of untreated blood were dried within 55 min, wipeability of all droplets originating from donors with prior anticoagulation treatment showed pronounced delays compared with the control, ranging from 20 min (ASA and clopidogrel + ASA) to 45 min (rivaroxaban). This pronounced effect was not seen in earlier studies, which might be explained by the higher volume of droplets used in this study, which resulted in a shift in relevance from drying to clotting processes. Significant differences between the drying times of the various anticoagulation regimes might be attributed to anticoagulant activity against different targets in the coagulation cascades. In conclusion, anticoagulation treatment prior to blood loss significantly affected the wipeability of blood stains. Anticoagulation therapy should therefore be taken into account in the analysis of blood stain patterns.