L8 - Bloodstain Pattern Analysis

Overview of Bloodstain Pattern Analysis (BPA)

  • Scope of BPA:

    • BPA is the study and analysis of blood stains to reconstruct crime scenes and establish the sequence of events leading to a bloodshed.

    • Common misconceptions about BPA being associated with violent crime dramas (e.g., CSI, Dexter).

  • Subjectivity in BPA:

    • Compared to DNA evidence (which uses statistical analysis), BPA relies heavily on the expert's opinion and interpretation, leading to variability based on individual expertise.

    • BPA faces scrutiny due to its subjective nature, drawing parallels to fingerprint analysis.

Characteristics of Blood

  • Biological Role of Blood:

    • Blood serves primarily to transport oxygen and remove waste within the body.

  • Blood Composition:

    • Blood consists of plasma and cellular components such as red blood cells, which are essential for interpreting stains.

  • Key Questions in Blood Identification:

    • Determining if a stain is indeed blood and whose blood it is.

    • Initial identification method:

    • ABC Method:

      • A: Appearance (Does it look like blood?)

      • B: Behavior (Does it behave like blood?)

      • C: Context (Is it typical for the scene of bloodshed?)

  • Example of Initial Identification Task:

    • Visual test comparing two dark stains; results demonstrate that one is blood (left) and the other is hair dye (right).

Confirmatory Tests for Blood

  • Presumptive Tests:

    • Test methods used to confirm blood presence, including HeamoStick and Hematrace as tools for identifying human blood specifically.

    • Presumptive Test Caveats:

    • False positives may occur; always record until confirmation.

    • High-dose hook effect leading to potential false negatives.

Bloodstain Pattern Classification

  • BPA as a Reconstruction Tool:

    • Bloodstains provide information about the event's circumstances, focusing on:

    • Size of stains

    • Shape

    • Distribution of blood

  • Importance of Stain Patterns:

    • Fingerprint and DNA can identify who was involved, while BPA explains how the incident transpired.

Classification of Bloodstains

  • The Four Main Categories:

    1. Gravity Patterns:

      • Stains caused by blood falling under gravity. (e.g., drip stains, drip trails)

    2. Contact Patterns:

      • Result from contact between blood and another surface, generating transfer, swipe, or wipe stains.

    3. Spatter Patterns:

      • Result from blood droplets being dispersed due to an external force applied to liquid blood. This includes impact stains and cast-off stains.

    4. Altered Patterns:

      • Stains altered through physical changes (e.g., dilution, voids).

Detailed Pattern Types in Each Category

  • Gravity Patterns:

    • Drip Stains: Round stains falling straight down (indicators of a stationary source).

    • Drip Trails: Multiple drip stains showing movement of a source.

    • Flow Patterns: Fluid movement of blood due to gravity, useful for determining movement and positions.

  • Contact Patterns:

    • Transfer Stains: Recognizable blood marks from contact.

    • Wipe Patterns: Altered stains created by an object moving through existing wet blood.

    • Swipe Patterns: A motion caused by transferring blood from one surface to another; often confused with wipe patterns.

  • Spatter Patterns:

    • Impact Stains: Smaller blood stains caused by an object striking blood.

    • Expiration Stains: Blood sprayed by air pressure from a wound or body (often seen in respiratory injuries).

    • Cast-off Stains: Blood released from an object moving in a particular direction, indicating the motion and movement of origins.

  • Altered Patterns:

    • Diluted Stains: Typically indicative of attempted cleanup, possibly reduced visibility or intensity of blood color.

    • Void Patterns: Absences of blood within existing patterns, implying presence of other objects or beings in proximity during the incident.

Principles of BPA Interpretation

  • Scientific Method in BPA:

    • Foundation on physics, biology, and mathematics ensures reliable outcomes.

    • Emphasizes fluid dynamics: the behavior of liquids in motion influenced by forces of gravity, impact, and movement.

  • Angles and Trajectories:

    • Blood travel is analyzed primarily through angles of impact, affecting shapes and perceived movement direction.

    • Use trigonometry for categorization and understanding patterns’ origins.

Reconstruction Using BPA

  • Two-Dimensional Reconstructions:

    • Area of convergence identified where multiple bloodstains intersect can inform about the original location of the blood source.

    • The area of origin can be calculated based on angles of bloodstains against the intersection point.

  • Order and Sequence of Events:

    • Identification of sequence based on overlapping stains and alterations, alongside the wetness of blood during events.

    • Consideration of external factors, such as drying rates, can affect interpretation of events.