Preface to Blood Traces: Interpretation of Deposition and Distribution

Why do we need another text on the interpretation of blood traces? Has not the subject of bloodstain pattern analysis been covered adequately in other books? We are of the opinion that there are many aspects of the interpretation of blood traces that are of concern and have not been treated with the thought or caution that should be afforded them. Most important, existing texts do not sufficiently emphasize the need for interpretations to be made by experienced scientists with strict adherence to the scientific method. At one point in the report issued by the National Research Council (NRC) of the National Academy of Sciences (NAS) Strengthening Forensic Science in the United States: A Path Forward published in 2009, the question is raised as to whether or not there is a scientific basis behind opinions offered in court in what has been termed bloodstain pattern interpretation.

Although there is a professional society of bloodstain pattern analysts, the two organizations that have or recommend qualifications are the IAI and the Scientific Working Group on Bloodstain Pattern Analysis (SWGSTAIN). SWGSTAIN's suggested requirements for practicing bloodstain pattern analysis are outwardly impressive, as are IAI's 240?hours of course instruction. But the IAI has no educational requirements for certification in bloodstain pattern analysis. This emphasis on experience over scientific foundations seems misguided, given the importance of rigorous and objective hypothesis testing and the complex nature of fluid dynamics. In general, the opinions of bloodstain pattern analysts are more subjective than scientific.

(NAS Report 2009, page 178)

As of the writing of this book, there are still no science-based higher education requirements for bloodstain pattern analysts, although efforts are underway in the Organization of Scientific Area Committees (OSACs) and the American Standards Board (ASB) of the American Academy of Forensic Sciences (AAFS). What fails to be appreciated in current practice and publications is that situations involving the interpretation of blood trace configurations are often very complex and the difficulties faced in rendering conclusions in this area are among the most scientifically challenging of those in any area of forensic science.

After a brief survey of the history of the subject of blood traces, several key issues will be identified in the introduction and ultimately dealt with in more detail later in the text. At the end of this chapter, some basic principles of the interpretation and analysis of the configuration of blood traces will be enunciated. These will be discussed at greater length and reiterated in subsequent chapters.

What are blood traces? Why use this as the title for our book rather than the generally accepted term in the English language of "bloodstain patterns?" The early contributors to nascent forensic science, including Edmund Locard, Hans Gross, and Eduard Piotrowski, used the term traces (traces de sang in French) or in the case of the German-speaking authors "Blutspuren" (Piotrowski 1895), the German expression meaning "traces of blood," when referring to the analysis of the physical configurations of blood evidence discovered at the scene of a crime. In addition, not all blood deposits stain the substrate and thus are not truly "stains." Examples would include dried deposits on hydrophobic surfaces, such as polymer coatings (e.g., paint and varnish), plastics, or treated glass, from which they can easily be dislodged. There is the potential for ambiguity with the term blood traces in that it could suggest biochemical genetic testing rather than an interpretation of the three-dimensional geometry of the deposits at a scene. Ultimately, no expression is ideal. However, the authors feel that a return to this original terminology of blood traces more accurately represents all that is encompassed in the interpretation of the deposition and distribution of blood evidence at a crime scene and hope that it will inspire others to recognize that it is more than just pattern classification and analysis.

Why is the term "bloodstain pattern analysis" a poor term for this work? There are several reasons for an objection to this terminology. First, it is not individual patterns that are being analyzed, but rather the totality of the physical aspects of the blood traces. Various blood trace configurations are necessarily examined and contribute to the overall reconstruction. However, the so-called patterns should not be the focus of the forensic examination. Assigning a name to the geometric features of a blood trace, i.e., classifying the pattern, does not necessarily advance the goal of reconstruction and trace interpretation of blood evidence. The interrelation of blood traces with respect to the scene or other objects bearing traces of blood is of much greater importance. Second, there is not a finite number of "bloodstain patterns," thus a crime scene scientist will undoubtedly encounter important geometric configurations that do not fit into a neat classification scheme. For example, blood that has soaked through a fabric or some other porous structure that is no longer present at the time of the scene investigation may represent the vestige of an earlier and more complex configuration, such as blood through bedding that has been disturbed or removed. Third, in the physical evidence context, the term pattern connotes a comparison process, whereby a questioned pattern is compared to a known pattern directed toward source attribution. Although this is the goal when analyzing fingerprints, footwear outsole imprints, tire tread impressions, toolmarks, firearm-produced marks on ammunition components, etc., this is not the central activity of the interpretation of blood traces. Fourth, the common usage of the term pattern indicates a repeating design (such as a textile, wallpaper, or floor pattern), which is certainly not the meaning for blood trace deposits. A recent focus on bloodstain pattern taxonomies in the OSAC Bloodstain Pattern Analysis subcommittee is misdirected, and instead the emphasis should be on providing an understanding of the interrelationship of the totality of blood trace deposits with the goal of reconstructing critical details of the event(s). Despite the expression "bloodstain pattern" having the previously identified weaknesses, we will sometime use this terminology in places in the text because the reader should be familiar with its existing and common usage.

What are traces? When we are confining our discussing to material evidence, there are two distinct conceptualizations for the term trace in English (De Forest 2001). The first refers to size or amount, where a trace of material indicates a small amount or a low concentration of a component in a larger specimen. An example of the concentration connotation would be trace elemental analysis. Despite the narrower "amount" or "concentration" focus being common in American usage for the term trace, it is not the defining characteristic of the word as it was intended by pioneers Edmund Locard and Paul Kirk. Instead, the term trace in the context used here more appropriately represents evidence of a prior presence or a vestige remaining after a causal object has been removed, as in the phrase "he vanished without a trace." This is consistent with one of the foundational philosophies of forensic science, Locard's exchange principle, which has been commonly, but somewhat inaccurately, reduced to the phrase "every contact leaves a trace." In this context, the term trace refers to the exchange of material or production of a pattern as a result of an interaction from a prior presence or both.

Trace evidence investigations are concerned with the goal of shedding light on an event by taking advantage of the marks that were made and/or material that was transferred or deposited during its occurrence. Unfortunately, blood and other physiological fluids are not commonly considered to be a type of trace evidence, however conceptually it is useful to consider them as such. The reason for the separate consideration of blood traces from trace evidence is because in common practice there exists a self-contained set of laboratory analytical techniques for such biological evidence. Despite this pigeonholing, blood evidence in the context of analysis of the deposition and distribution of blood traces is clearly a form of trace evidence.

Blood trace deposits, often present at scenes of violence, may arguably be one of the most important types of physical evidence in a scene investigation. Despite this, other complementary traces may be present and should be thoroughly considered in a holistic approach to scene investigation. The analysis of blood traces may provide the identification of its origin (species) and subsequent approach to individualization (DNA). Additionally, the analysis of blood may answer questions that extend beyond...