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Practice Exams

Module 7.12: Blood Pattern Analysis & Serology

I. Serology: The Identification of Blood

Before any advanced analysis can be done, a red-brown stain found at a crime scene must be definitively identified. This involves a three-step process.

Step 1: Is it blood? (Presumptive Screening Tests)

  • These are highly sensitive tests that indicate the possible presence of blood. They rely on the peroxidase-like activity of hemoglobin in the blood.
  • Phenolphthalein Test (Kastle-Meyer Test): A drop of phenolphthalein reagent and a drop of hydrogen peroxide are added to the stain. An immediate bright pink color indicates a positive result.
  • Luminol Test: A chemical spray used to search for latent (invisible) bloodstains, often on cleaned surfaces. In a darkened room, luminol will glow a faint blue when it comes into contact with blood.
  • Limitation: These tests can produce false positives. Other substances like horseradish, rust, and some chemical cleaners can also cause a positive reaction.

Step 2: Is it blood? (Confirmatory Tests)

  • If the screening test is positive, a confirmatory test is performed to prove conclusively that the stain is blood.
  • Takayama Test / Teichmann Test: These are microcrystal tests. A small amount of the stain is treated with specific chemicals. The formation of characteristic crystals, when viewed under a microscope, confirms the presence of blood.

Step 3: Is it human blood? (Precipitin Test)

  • Once confirmed as blood, it is necessary to determine if it is of human or animal origin.
  • The Precipitin Test is the standard method. It is based on an antigen-antibody reaction.
    1. An extract of the bloodstain (containing human antigens, if it's human blood) is placed in a test tube.
    2. Human antiserum (produced by injecting a rabbit with human blood, which then creates antibodies to it) is carefully added on top.
    3. If the blood is human, a cloudy ring or precipitate will form at the interface of the two liquids. This confirms the blood is of human origin.

Summary of Serology Process:

  1. Presumptive Test (e.g., Kastle-Meyer): Is it probably blood?
  2. Confirmatory Test (e.g., Takayama): Is it definitely blood?
  3. Precipitin Test: Is it human blood?

Once identified as human blood, the sample would then proceed to the DNA lab for profiling (as covered in Module 7.5).

III. Classification of Bloodstain Patterns

Bloodstain patterns are generally classified into three main categories: passive, spatter, and transfer.

A. Passive Stains

  • These are stains created by the force of gravity alone.
  • Drips: Individual drops of blood falling onto a surface.
  • Pools: Blood that has collected on a surface.
  • Flows: Patterns created by the movement of blood on a surface due to gravity.

B. Spatter Stains

  • These are stains that are created when an external force other than gravity is applied to liquid blood. The size of the droplets is related to the amount of force.

  1. Impact Spatter (Low, Medium, and High Velocity):

    • Low-Velocity Spatter (LVIS): Large droplets (4mm or more). Produced by minimal force, such as blood dripping from a wound.
    • Medium-Velocity Spatter (MVIS): Droplets of 1-4mm. Typically associated with blunt force trauma (beatings) or stabbings.
    • High-Velocity Spatter (HVIS): Very fine droplets or mist (less than 1mm). Associated with high-energy forces like gunshots (forward spatter from the exit wound, back spatter from the entrance wound) or explosions.

  2. Cast-Off Spatter: Created when a bloody object (like a weapon or a fist) is swung. The blood flies off the object in an arc, often creating a linear pattern on a ceiling or wall. The number of cast-off patterns can suggest the minimum number of blows delivered.

  3. Arterial Spatter: The characteristic pattern created when a major artery is breached. The blood is projected in large, arcing patterns due to the pumping of the heart.

  4. Expirated Spatter: Blood that is coughed or blown out of the nose or mouth. It often contains air bubbles and can be mixed with saliva.

C. Transfer Stains

  • These are stains created when a wet, bloody surface comes into contact with another surface.
  • Wipes: A pattern created when an object moves through an existing bloodstain.
  • Swipes: The transfer of blood from a moving bloody object onto a clean surface.
  • Pattern Transfers: A recognizable image of a bloody object is left on a surface, such as a bloody handprint, footprint, or the impression of a weapon.

Introduction

Blood is one of the most common and important types of physical evidence found at violent crime scenes. Its analysis involves two distinct but related disciplines: Serology, which deals with the identification of bodily fluids, and Bloodstain Pattern Analysis (BPA), which is the interpretation of the shape, size, and distribution of bloodstains to reconstruct the events of a crime. This module will cover the basics of serology—confirming a substance is blood and determining its origin—and then delve into the fascinating field of BPA, which can tell a story written in blood.

Key Learning Objectives:

  • Differentiate between Serology and Bloodstain Pattern Analysis.
  • Describe the process for identifying a substance as blood.
  • Explain how to determine if blood is of human or animal origin.
  • Understand how the shape of a bloodstain reveals its angle of impact.
  • Identify and interpret different types of bloodstain patterns (passive, spatter, transfer).

II. Bloodstain Pattern Analysis (BPA)

BPA is the examination of the shapes, locations, and distribution patterns of bloodstains, in order to provide an interpretation of the physical events that gave rise to their origin. A bloodstain pattern analyst can reconstruct events by applying principles of biology, physics (fluid dynamics), and mathematics.

A. Physical Properties of Blood

  • Blood is held together by surface tension, which causes it to form spherical droplets when in flight.
  • A standard drop of blood has a volume of approximately 0.05 mL.

B. Information from a Single Bloodstain

  1. Shape and Angle of Impact: The shape of a bloodstain reveals the angle at which it struck a surface.

    • 90-degree impact (drip): Produces a perfectly round stain.
    • Less than 90 degrees: The stain becomes elongated, forming an elliptical or "teardrop" shape. The narrower the angle, the longer the stain.
    • The pointed or "tailed" end of the bloodstain always points in the direction of travel.

  2. Calculating the Angle of Impact:

    • The angle can be calculated using trigonometry: Angle (θ) = arcsin (Width / Length)
    • For example, a stain that is 1 cm wide and 2 cm long struck the surface at an angle of 30 degrees (arcsin(1/2) = 30°).

C. The Area of Convergence and Origin

  • Area of Convergence: The point on a two-dimensional surface from which the bloodstains originated. It is found by drawing straight lines through the long axis of several individual bloodstains. The point where the lines intersect is the area of convergence.
  • Area of Origin (or Point of Origin): The three-dimensional location from which the blood originated. It is determined by using the angle of impact of each stain in conjunction with the area of convergence. This helps to determine if a victim was standing, sitting, or lying down when they were bleeding.

Conclusion

Blood evidence provides a wealth of information to the criminal investigator. Through serology, a stain can be identified as human blood, linking it to a potential victim or suspect via DNA. Through bloodstain pattern analysis, the physical properties of blood can be used to reconstruct the violent events of the crime scene. The patterns left behind can reveal the type of weapon used, the number of blows struck, the position of the victim and assailant, and the sequence of events. Together, these two disciplines turn a chaotic scene of violence into a readable narrative, allowing the blood itself to speak for the victim.