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6.3 Blood Pattern Analysis

6.3.1 Introduction

• Blood evidence is rich in information beyond biochemical composition.

• Blood operates as a fluid under pressure throughout the body, and its physical behavior is controlled by predictable natural forces.

• Blood distribution can provide clues about:

  • Where the assault occurred.

  • How the injury occurred.

  • Subsequent actions during and after the attack.

• Observation of bloodstain patterns is crucial for crime scene investigations, focusing on the

  • Where,

  • What,

  • How of the incident rather than identifying who was present.

• The typical human possesses between 4 and 6 liters of blood, allowing for significant amounts at violent scenes.

• Blood is denser than water, with roughly three times its viscosity.

  • Viscosity: Resistance of a liquid to flow through a capillary under pressure.

  • Example: Water has low viscosity; motor oil has high viscosity.

• Surface tension: Behavior of surface molecules forming an elastic membrane, minimizing exposed area.

  • Influences the shape and dispersion of blood droplets.

• Observations from bloodstains can yield the following insights:

  1. Locations and distribution suggest:

  • Crime location

  • Positions of victim and assailant(s)

  • Locations of nearby objects

  • Movements post-attack.

  1. Formation manner (e.g., passive, active, transfer).

  2. Blood droplet origin and travel direction.

  3. Bloodstain patterns analysis can reveal:

  • Wound origin on the body.

  • Weapon type and number of impacts.

  • Relative positioning of people involved.

  • Sequence of events.

  1. The type of weapon used, its velocity, and minimum impact number.

  2. Verification of suspect/witness/victim accounts.

• Caveat: Bloodstain analysis often provides non-conclusive but corroborative scenarios.

6.3.2 Bloodstain Patterns Analysis

• Initial forensic task when bloodshed is known or suspected is to identify all blood droplets at the scene.

  • Blood locations are typically visible but may be absent due to cleaning or movement of objects.

  • Tools like luminol and fluorescein can help locate blood in low-trace conditions.

• Bloodstain pattern analysis (BPA) integrates multiple scientific fields to interpret blood patterns at crime scenes.

• Bloodstains can be classified into three categories:

  1. Passive bloodstains:

  • Formed solely by gravity.

  • Subcategories:

    • Passive drops: Simple droplets falling.

    • Drip patterns: Droplets falling into existing blood.

    • Flow patterns: Movement influenced by gravity or object movement.

    • Pool patterns: Pooled blood stationary for a time.

  • Observing these patterns can indicate body movement and flow direction.

  1. Active bloodstains:

  • Formed by additional forces.

  • Originate from an attack, flung blood, or blood pressure from the body through a wound.

  • Classification based on the impact velocity of the impacting object:

    • Low-velocity impact spatter: Up to 5 ft/s (1.5 m/s)

      • Large droplets (3-4 mm) typically from slow attacks (e.g., fist).

    • Medium-velocity impact spatter: 5 to 100 ft/s (1.5 to 30 m/s)

      • Droplet sizes between 1-4 mm from blunt/sharp force injuries.

    • High-velocity impact spatter: Above 100 ft/s (>30 m/s)

      • Fine mist of droplets under 1 mm from sources like bullet impacts, explosions, or high-speed machinery.

  • Blood patterns forced from the mouth/nose are called expirated patterns, identifiable by the presence of the amylase enzyme and cheek cells.

  • Patterns can also indicate arterial injuries, characterized by an inverted V-shape reflecting blood pressure changes.

  1. Transfer stains:

  • Leave imprint information (e.g., footprints, handprints).

• Proper collection and preservation methods are critical for blood-containing evidence.

Bloodstains and Their Characteristics

• The nature of the surface where blood lands affects the appearance of stains:

  • Smooth surfaces (glass, linoleum): Produce sharp-edged, well-defined stains.

  • Rough surfaces (carpeting, concrete, wood): Create distorted stain patterns.

• Bloodstains often appear as spherical droplets in transit, influenced by surface tension.

• Blood droplets impact angles and surface characteristics shape the final pattern.

• Careful sizing and observation are crucial for interpreting evidence.

Angle of Impact

• To determine the angle of impact, measure the width and length of the bloodstain.

  • Use trigonometry: $ext{sin}( heta) = rac{W}{L}$

  • Where:

  • $W$ = width of stain

  • $L$ = length of stain

• This formula provides an approximation, assuming droplets travel in straight lines, though actual paths may be influenced by other factors.

• Carefully drawn lines backtrack droplets to assess the area of convergence - likely the origin of blood spatter.

BOX 6.8 Brief on Blood Impact Analysis

1. Bloodstains are categorized as:

   • Passive

   • Active

   • Transfer

2. Passive bloodstains form solely under gravity’s influence.

3. Active bloodstains arise through forces besides gravity and are divided based on impact velocity:

   • Low, medium, high velocity.

4. Transfer stains provide imprint information.

5. Proper collection is vital for blood-containing evidence.