Blood Evidence and Pattern Analysis Dry Lab Notes

Trajectory and Impact Angle

  • To determine the impact angle of a blood droplet, calculate the arcsine (inverse sine) of the width divided by the length of the elliptical droplet.

  • The formula is: angle = sin^{-1}(width/length)

  • For example, if a blood droplet is 2mm wide and 5mm long:

    • 2/5 = 0.4

    • sin^{-1}(0.4) ≈ 23.5°

    • Therefore, the impact angle is approximately 23 degrees.

Directionality of Stains

  • When analyzing bloodstains at a crime scene, it is important to determine the direction of travel of the blood droplets.

Documenting Directionality and Pattern Type

  • To properly document directionality and pattern type:

    • Examine each surface independently.

    • Consider commonality between surfaces, as patterns may continue from one surface to another.

Cast-Off Pattern Recognition

  • Cast-off patterns are commonly encountered in cases involving blunt force trauma and sharp force trauma.

  • Counting the number of swings can be estimated (+/- 2).

  • The weapon in motion causes blood to fly off tangentially along the path of the swing.

  • The consistency of the cast-off pattern changes with the arc of the swing.

Perpetrator Considerations

  • During violent actions, the perpetrator is not immune to having blood spattered onto them.

Blood Pattern Clues

  • Blood patterns can be direct imprints of the tool or object used.

  • They can also provide clues about body parts or materials involved.

Presumptive Tests

  • Presumptive tests can detect whole blood at dilutions up to 1:2,000,000.

  • As few as 250 red blood cells (RBCs) are required for a positive result.

Luminol

  • Luminol is rarely used for visible blood due to it being a health hazard.

  • Luminol can greatly enhance latent blood patterns, even after bleaching or washing.

  • Tests include:

    • Blank Water

    • Positive-known blood

    • Negative= no blood

  • OBTI Test