OSAC 2022-S-0030 Standard for 

Methodology for Bloodstain Pattern Analysis 

1

OSAC 2022-S-0030 

Standard for Methodology 

in Bloodstain Pattern 

Analysis

Bloodstain Pattern Analysis

Physics / Pattern Interpretation Scientific Area Committee

Organization of Scientific Area Committees (OSAC) for Forensic Science
 OSAC 2022-S-0030 Standard for 

Methodology for Bloodstain Pattern Analysis 

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Draft OSAC Proposed Standard

OSAC 2022-S-0030 Standard

Methodology for 

Bloodstain Pattern Analysis

Prepared by 

Bloodstain Pattern Analysis Subcommittee

Version 2.0

May 2023

Disclaimer:

This OSAC Proposed Standard was written by the Bloodstain Pattern Analysis Subcommittee of 

the Organization of Scientific Area Committees (OSAC) for Forensic Science following a 

process that includes an open comment period. This Proposed Standard will be submitted to a 

standards developing organization and is subject to change. 

There may be references in an OSAC Proposed Standard to other publications under 

development by OSAC. The information in the Proposed Standard, and underlying concepts and 

methodologies, may be used by the forensic-science community before the completion of such 

companion publications.

Any identification of commercial equipment, instruments, or materials in the Proposed Standard 

is not a recommendation or endorsement by the U.S. Government and does not imply that the 

equipment, instruments, or materials are necessarily the best available for the purpose.

To be placed on the OSAC Registry, certain types of standards first must be reviewed by a 

Scientific and Technical Review Panel (STRP). The STRP process is vital to OSAC’s mission of 

generating and recognizing scientifically sound standards for producing and interpreting forensic 

science results. The STRP shall provide critical and knowledgeable reviews of draft standards or 

of proposed revisions of standards previously published by standards developing organizations 

(SDOs) to ensure that the published methods that practitioners employ are scientifically valid, 

and the resulting claims are trustworthy.

The STRP panel will consist of an independent and diverse panel, including subject matter 

experts, human factors scientists, quality assurance personnel, and legal experts, which will be 

tasked with evaluating the proposed standard based on a comprehensive list of science-based 

criteria.
 OSAC 2022-S-0030 Standard for 

Methodology for Bloodstain Pattern Analysis 

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For more information about this important process, please visit our website 

at: https://www.nist.gov/topics/organization-scientific-area-committees-forensicscience/scientific-technical-review-panels. 
 OSAC 2022-S-0030 Standard for 

Methodology for Bloodstain Pattern Analysis 

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1. Introduction

This document uses the following verbal forms: “shall” indicates a requirement, “should” 

indicates a recommendation, “may” indicates permission, and “can” indicates a possibility or 

capability. 

2. Scope

2.1 This document covers the methodology for bloodstain pattern analysis. It does not 

address specific methods but rather the systematic approach for bloodstain pattern 

analysis casework. For instance, it addresses when to classify but not how to classify 

patterns. This methodology defines the six overall standardized and chronologically ordered 

steps to perform bloodstain pattern analysis. Each step provides direction for the 

development of data supported interpretations. Specifically, this methodology involves the 

examination of bloodstains and bloodstain patterns to determine the significance of their 

presence or absence, what potential forces or mechanisms could result in those 

bloodstains or bloodstain patterns, and the significance of those determinations in regard 

to the scope or reason for the examination. 

2.2 To limit the role of contextual bias and improve bloodstain pattern analysis decision 

making, a linear sequential unmasking-expanded[1] approach is incorporated in this 

methodology to manage exposure to task-irrelevant data by optimizing the order of 

information processing.

2.3 This document does not address documentation, preservation, or collection at the 

scene or on items of evidence.

3. Normative References 

There are no normative reference documents. Annex A, Bibliography, contains informative 

references.

4. Terms and Definitions

For the purposes of this document, the following terms and definitions apply.

4.1 Analyst: An individual who has successfully completed the prescribed course of 

study (BPA). 

4.2 Assumption: Something supposed as true, without adequate supporting evidence or 

certain knowledge.

4.3 Cognitive Bias: A set of influences that may affect the reliability and validity of 

one’s observations and conclusions.

4.4 Event Segment: Time snapshot depicted by moments of activity. 
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Methodology for Bloodstain Pattern Analysis 

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4.5 Examination: The act or process of observing, searching, detecting, recording, 

prioritizing, collecting, analyzing, measuring, comparing, and/or interpreting.

4.6 Interpretation: Explanation for the observations, data, and calculations.

4.7 Mechanism: The action or event process resulting in a bloodstain or bloodstain 

pattern.

4.8 Methodology: The analytical processes and procedures used to support bloodstain 

pattern analysis.

4.9 Observed Data: Observable features within a bloodstain or blood stain pattern. 

4.10 Opinion: View, judgement, or belief that takes into consideration other information 

in addition to observations, data, calculations, and interpretations.

4.11 Post-incident Artifact: Any post-occurrence alteration to a bloodstain or bloodstain 

pattern resulting from the actions of first responders, medical personnel, or an individual.

4.12 Task-irrelevant Information: Information that is not necessary for making 

interpretations about the propositions in question, or that it assists only in making 

interpretations from something other than the physical evidence designated for testing or 

assists only in drawing conclusions by some means other than an appropriate analytic 

method.

4.13 Task-relevant Information: Information that is necessary for making interpretations 

about the propositions in question, from the physical evidence that has been designated 

for examination, or through the correct application of an accepted analytic method by a 

competent analyst.

4.14 Utility: The usefulness of a bloodstain or bloodstain pattern for a further step in the 

analysis.

5. Requirements

Analysts will be exposed to various sources and types of information throughout their analysis 

which may be task-relevant or task-irrelevant.1 When considering how this information can 

influence their decision-making, they shall consider and document the potential for cognitive 

bias. Where possible, this methodology should follow linear sequential unmasking – expanded.2

The analyst shall access information in the following order prescribed by the methodology. If 

information is accessed out of order, it shall be documented and explained.

1 National Commission on Forensic Science, Ensuring the forensic analysis is based upon task-relevant 

information. It is available at https://www.justice.gov/archives/ncfs/page/file/641676/download

2 Dror, Itiel E, and Jeff Kukucka. “Linear Sequential Unmasking-Expanded (LSU-E): A general approach 

for improving decision making as well as minimizing noise and bias.” Forensic science international.

Synergy vol. 3 100161. 13 Aug. 2021. It is available at https://doi.org/10.1016/J.FSISYN.2021.100161.
 OSAC 2022-S-0030 Standard for 

Methodology for Bloodstain Pattern Analysis 

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Prior to the start of analysis, the information available to be utilized shall be defined and a list of 

these source materials created. If a customer has made a request regarding the purpose and focus 

of the analysis, this request shall be included in the case documentation. 

A request for analysis may be undefined, where much is unknown. In these cases, the analyst 

shall render opinions based upon the quality and quantity of the information available for 

analysis. 

If the request is to evaluate a statement involving specific positions or actions, the analysis shall 

be completed prior to learning the focus of that statement. Once analysis is complete, the 

opinions rendered can be compared to support or refute the statements made.

5.1 Observation and Data Collection

The analyst shall utilize imaging (e.g., photographs, 3D-laser scanning) and notes to 

document observations of bloodstained items and bloodstained areas to characterize stain 

and pattern size, shape, distribution, appearance, and location. Sketches or diagrams may 

also be utilized to aid documentation. It is understood that imaging may be provided to 

the analyst and/or produced by the analyst. 

5.1.1 Locate all areas where blood is present.

5.1.2 Locate all areas where blood is notably absent.

5.1.3 Characterize stain and pattern sizes. 

 5.1.3.1 If measuring individual stains, measurements shall be in metric.

 5.1.3.2 For spatter stains, sizes of stains refer to the widths.

 5.1.3.3 For non-spatter stains, sizes refer to overall dimensions.

5.1.3.4. Measurements for the overall dimensions of the stains grouped 

into patterns may also be documented. 

5.1.4 Characterize stain and pattern shapes.

 5.1.4.1 Circular

 5.1.4.2 Elliptical

 5.1.4.3 Irregular

 5.1.4.4 Linear or curvilinear

5.1.4.5 Edge characteristics, to include observable characteristics such as 

clearly defined sharp edges, spines, or feathering.

5.1.5 Characterize stain distribution.
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Methodology for Bloodstain Pattern Analysis 

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5.1.5.1 Stain arrangement: radiating, linear, curvilinear, etc.

5.1.5.2 Size distribution within a pattern

5.1.5.3 Distribution of aspect ratios of ellipses fitted to the stains (circular 

to elliptical)

5.1.5.4 Angular distributions of the main axes of the stains (directionality)

5.1.5.5 If stains within a pattern converge to an area

5.1.5.6 2D or 3D deposition – e.g., located on the top surface only or 

penetrating the target substrate

5.1.5.7 Density of stains within the pattern

5.1.6 Characterize stain and pattern appearance

5.1.6.1 Coloration, dilution, thickness 

5.1.6.2 Wet, dry, flaking

 5.1.6.3 Clotted

 5.1.6.4. Altered

 5.1.6.5 Flow, gravity effects

 5.1.6.6 Impression patterns: Friction ridge detail, footwear

5.1.6.7 Vacuoles/bubble rings and mucous strands

 5.1.6.8 Repeating pattern of the same characteristics

5.1.7 Characterize the surface the stains are present on. 

 5.1.7.1 Fixed versus moveable objects 

5.1.7.2 Porous, non-porous

● Construction and composition of fabrics and textiles

5.1.7.3 Smooth, irregular, textured, treated, flexible, etc. 

 5.1.7.4 Curved, flat, angled

5.1.8 Document other observable material e.g., tissue, hair, bullet holes

5.1.7 Characterize the physical location of stains
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Methodology for Bloodstain Pattern Analysis 

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5.2 Designate areas of blood into groupings of stains/patterns 

5.2.1 Organize patterns by stain size, shape, distribution, appearance, and 

location.

5.2.1.1 Not all bloodstains can be organized into patterns because their 

observable characteristics do not provide enough information to proceed.

5.2.2 Determine the utility of bloodstains or bloodstain patterns. A utility 

determination can be decided based on a multitude of reasons.

5.2.2.1 The observed characteristics of the bloodstains and bloodstain 

patterns may have ambiguous information that can be attributed to 

multiple pattern types and the determination is made that a classification is 

not possible. 

5.2.2.2 The lack of quantity of stains present may not allow the analyst to 

definitively exclude any mechanism or make any meaningful 

classification. 

5.2.2.3 The bloodstains or bloodstain patterns may exhibit characteristics 

of alteration to an extent that the original deposition cannot be reliably 

classified.

5.2.2.4 The bloodstains or bloodstain patterns may be deposited on a 

surface that would result in unreliable characteristics leading to unreliable 

classifications due to the interaction that the blood and target surface 

would have.

5.2.2.5 The bloodstains or bloodstain patterns appear to be overlapping in 

such a way that it cannot be reliably determined which stains are 

associated and which stains are not associated. 

5.3 Classification of Patterns from Observable Characteristics

Classification of bloodstains and bloodstain patterns is the process that utilizes 

observable characteristics to include those mechanisms that cannot be excluded, 

resulting in an interpretation. At this step/stage the analyst shall only use observable 

stain(s) and pattern characteristics to make their initial interpretation(s). 

5.3.1 The analyst should consider potential mechanisms or means of 

deposition that can be excluded. The analyst shall then note all pattern types that 

will be given further consideration during the refinement process. Pattern types 

can be assigned based on mechanism or force and can be as definitive as using 

the ASB BPA terminology or generally describing a mechanism or force (e.g. 

gravity, airborne, or contact).
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Methodology for Bloodstain Pattern Analysis 

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5.3.2 Additional forensic analysis, such as forensic biology and DNA testing, 

may assist in refinement of the pattern classification and the logical exclusion of 

mechanisms. The analyst should determine if collected samples will be submitted

for this forensic analysis.

5.3.3 Where applicable, the analyst may utilize methods to apply to areas of 

spatter patterns to aid in determining flight path characteristics, area of origin, 

area of convergence, etc.

5.3.4 The analyst should be able to justify why a stain/pattern has not been 

classified. The value and potential significance of these stains/patterns will be 

assessed based upon the observed data and/or task-relevant information.

5.4 Refinement of Interpretations Applying Task-relevant Contextual Information

Refinement is the logical exclusion of pattern types included in the interpretations above 

with the addition of forensic biology/DNA and medical/pathology reports. This 

refinement is relevant when assessing whether to exclude impact, expiration, forward and 

backspatter, cast-off, or projected patterns. Examples of refinement which can be made 

based on review of this task-relevant contextual information:

● If there is no pathological evidence of a gunshot injury, backspatter and 

forward spatter may be excluded as possible pattern types.

● If the source of blood has been narrowed to a single deceased individual 

and there is no evidence of blood in that person's airway from autopsy 

reports, then expiration from that person can be eliminated as a possible 

mechanism.

5.4.1 The analyst shall only consider information relevant to source and/or 

blood-letting injuries necessary for pattern interpretation refinement. The analyst 

shall document and apply this information to eliminate pattern types not 

supported by blood-letting injuries. 

5.4.2 Considering the results of the forensic analysis, the analyst should refine 

the previously documented list of potential mechanisms or means of deposition 

and further exclude any pattern types that are no longer logical given the results.

5.4.3 If the analyst becomes aware that any piece of this contextual information is 

potentially unreliable, the analyst shall return to the interpretations from 5.3 and 

not consider this information.

5.4.4 An analyst shall not include a pattern type based solely on contextual 

information.

5.5 Event Reconstruction/Case Interpretations
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Methodology for Bloodstain Pattern Analysis 

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Event reconstruction in bloodstain pattern analysis is the process of incorporating and 

assimilating all observed data and interpretations to begin to determine the possible scene 

events that created the pattern(s)/stain(s). All outcomes at this step are termed to be 

interpretations. The interpretations may be generic (e.g., an impact occurred in this 

location, or a transfer pattern is located on the floor) or specific to the case (e.g., The 

evidence of a drip trail spanning the area between the kitchen and the front door, which 

has been DNA sourced to [name], would indicate movement of a source of their blood 

between these two points). Reconstruction of events are first addressed in finite periods 

of time or event segments3 where specific patterns may be associated with specific 

events. It is also possible for these event segments to be sequenced relative to one 

another as part of the event reconstruction. It is understood that not every event segment 

can be determined and therefore, it is not expected that every stain or pattern can be or 

needs to be interpreted.

Each interpretation involves the generation of a hypothesis and testing the hypothesis by 

comparing predictions against observed data. A hypothesis is generated as a possible 

explanation for some initial set of information, and then tested by examining independent 

data or information that tests critical aspects of the hypothesis and/or differentiate it from 

alternative explanations. Analysts must consider alternative hypotheses and either 

eliminate them or refine the original hypothesis to include them. 

The analyst may apply additional case-specific, task-relevant information to inform their 

bloodstain pattern interpretations and hypotheses. The analyst shall document when the 

contextual information was accessed. Some of the contextual information may also 

contain task-irrelevant and biasing information. Analysts shall document the specific 

task-relevant information utilized and not utilize task-irrelevant information. When taskirrelevant information is accessed, the biasing power and objectivity of the information 

should be evaluated to mitigate its effects. In some cases, this additional case-specific 

information will not be needed at all. 

 Examples of additional contextual information include: 

● First responder actions

● Investigative reports

● Other relevant forensic reports (e.g., footwear comparison, patent latent 

print comparison)

5.5.1 The analyst shall determine and document all assumptions made and limitations that 

may have influenced interpretation.

 Examples:

● Only one source of blood was present at the crime scene.

● Only Person X has injuries consistent with a projected blood source.

● The deposition of all bloodstains occurred contemporaneous to the 

bloodletting event(s) in question.

3 Gardner, R. and Bevel T., 2009. Practical crime scene analysis and reconstruction. CRC Press.
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● Possible scene alteration is present from medical intervention.

● Clean-up activities occurred.

● Analysis was performed from photographs only and was limited to the 

documentation provided. 

5.5.2 Concurrent with 5.5.1, where applicable and reasonable, the analyst shall make 

interpretations in regard to the following with the understanding that not every stain or 

pattern needs to be interpreted as stated above:

 5.5.1.1 Interrelationship(s) of stains(s) and pattern(s) based on their 

locations (e.g., satellite patterns adjacent to projected patterns or impact 

patterns on adjacent walls in a corner of a room). This interrelationship 

can be based on the physical proximity of two or more patterns, 

recognized repeating patterns throughout the scene, or potential 

correlating mechanisms of patterns.

 5.5.1.2 Directionality of stains and patterns based upon their shapes and 

characteristics (e.g., within swipe and wipe patterns).

 5.5.1.3 Potential sources of blood that contributed to stains or patterns 

such as wounds, pools, and objects containing blood which may be 

responsible for stains and patterns. Where possible, correlate potential 

blood sources to stains and patterns with potential location of sources 

during the mechanistic event (e.g., incorporating area of origin 

calculations to wound locations). Where applicable, source associations 

shall incorporate results from available DNA reports to further aid in 

determining sources.

 5.5.1.4 Sequencing the order of deposition of stains and patterns

 5.5.1.5 Voids and how they may have been created (e.g., limiting angles)

 5.5.1.6 Repositioned objects/movement using voids or flow patterns

 5.5.1.7 Identifying patterns that could potentially be pre- or post-incident 

artifacts (e.g., insect stains, stains that appear to be very old, or medical 

personnel attempts to aid injured persons)

 5.5.1.8 Potential influence of environmental factors on blood deposition 

and patterns (e.g., temperature, humidity, or altered stains)

 5.5.1.9 Indications of time since deposition (e.g., perimeter stains, 

coloration, or serum separation)

5.6. Final Opinion 

5.6.1 Upon completing all interpretations, the analyst, where applicable, should 

render a final opinion regarding the request or investigative questions. This 
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Methodology for Bloodstain Pattern Analysis 

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opinion shall not use any unsupported data. Examples of unsupported opinions 

include:

● “The blood spatter on Mr. X’s clothing indicated he was less than two 

meters away from the victim at the time of impact.” 

● “Based upon the size of the bloodstain pattern on the bed, a minimum of 

two liters of blood was present.”

● “The presence of clotting within spatter stains showed that bleeding had 

commenced more than X minutes prior to impact.”

5.6.2 In cases where the request for bloodstain pattern analysis includes a specific 

question or statement analysis (e.g., a statement about what happened and 

bloodstain pattern analysis is used to support or refute such statements) the 

analyst shall evaluate all proposed possibilities, and document support for and 

support against them. The analyst should be actively looking for and trying to find 

support for alternative hypotheses. In some cases, a proposed possibility may not 

have sufficient data to make a determination. The analyst may determine that 

another hypothesis has more support based on the evidence and interpretations. In 

such cases, the analyst shall document the basis of this opinion. 
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Annex A

(informative)

Bibliography

1] Dror, I. E, and Kukucka, J., “Linear Sequential Unmasking-Expanded (LSU-E): “A general 

approach for improving decision making as well as minimizing noise and bias.” Forensic 

science international. Synergy vol. 3 100161. 13 Aug. 2021. It is available at 

https://doi.org/10.1016/J.FSISYN.2021.100161. 

2] Gardner, R. and Bevel T., 2009. “Practical crime scene analysis and reconstruction.” CRC 

Press.

3] National Commission on Forensic Science, “Ensuring the forensic analysis is based upon 

task-relevant information.” It is available at 

https://www.justice.gov/archives/ncfs/page/file/641676/download