Bloodstain Pattern Analysis in the Lab Setting

Bloodstain Pattern Analysis in the Lab Setting

Bloodstain pattern analysis (BPA) is the examination of the size, shape, distribution, and location of bloodstains to help reconstruct events at a crime scene. In the lab, you work with prepared panels rather than live scenes, but the principles, measurements, and reporting standards are identical. Accuracy in measurement and discipline in language are both non-negotiable.

Safety First

Even when training materials use simulated blood or an approved substitute, treat every fluid as a potential biohazard. Before touching any panel:

• Don nitrile gloves, safety glasses, and a lab coat.
• If aerosols are possible (e.g., any spraying activity), add a face shield and a minimum N95 respirator.
• Work on a surface lined with absorbent bench paper; dispose of all paper and gloves in a biohazard waste container after the session.
• Wash hands thoroughly after removing PPE.
• Notify your instructor immediately if you sustain any skin break or splash to eyes or mucous membranes.

Never eat, drink, or apply cosmetics in the lab area.

Bloodstain Pattern Categories

The Scientific Working Group for Bloodstain Pattern Analysis (SWGSTAIN) published terminology guidelines that define the major pattern categories. You should know these cold.

Passive patterns result from gravity alone. A drop falling straight down forms a circular stain; falls from greater height produce more pronounced scalloping around the edge (crenation).

Projected patterns result from force applied to a blood source. Arterial spurting, cast-off arcs from a bloody object in motion, and impact spatter from a blow or gunshot all fall here.

Transfer patterns occur when a bloody surface contacts another surface. A wipe is produced when an object moves through an existing stain; a swipe is produced when a bloody object moves across a surface. Fingerprints, palm prints, shoe impressions, and fabric impressions are transfer patterns.

Satellite stains are smaller droplets that break away from the primary drop on impact. Their presence and distribution give information about surface texture and drop velocity.

Stain Geometry and Impact Angle

An airborne blood droplet is roughly spherical. When it strikes a flat surface at an angle less than 90°, it distorts into an ellipse. The relationship between the ellipse axes is:

> sin(α) = W ÷ L

where α is the impact angle, W is the measured width (short axis), and L is the measured length (long axis), both in the same units.

Measure both axes with a metric ruler to the nearest millimeter. Use the arcsin (sin⁻¹) function on your calculator. A stain that is nearly circular has an angle near 90° (steep impact); a very elongated stain has a low angle (shallow, glancing impact).

Finding the Area of Origin

The area of origin is the three-dimensional region in space from which the blood was dispersed. The two-dimensional point of convergence is found first: draw directional lines (using the long axis of each elliptical stain, extended back toward the tail) for at least eight stains. The area where those lines cluster is the point of convergence on the surface.

Stringing method: Pin a length of string at each stain, angle it upward at the calculated impact angle (using a protractor), and run it toward the convergence. The intersection of strings in three-dimensional space approximates the area of origin.

Trigonometric method: Measure the horizontal distance (D) from each stain to the point of convergence on the floor or panel surface. The estimated height (H) of origin above that surface is:

> H = D × tan(α)

Calculate H for each stain and average the results. Report the result as a range, not a single point, because measurement error accumulates.

Qualified Language and Report Writing

SWGSTAIN guidelines require analysts to use language that honestly reflects the limits of the method. Do not write "the blow was struck at exactly 1.2 m above the floor." Instead write "the area of origin is estimated to be between 0.9 m and 1.4 m above the floor, based on trigonometric analysis of eight impact spatter stains."

Your written report must include:

1. Purpose and scope — what panel was examined and why.

2. Methodology — how measurements were made and which formula was applied.

3. Measurements and calculations — a table of all stain data.

4. Findings — pattern classifications and area-of-origin estimate with units.

5. Sketch — a scaled diagram of the panel showing stain locations, convergence lines, and estimated origin.

6. Limitations — sources of error, surface irregularities, or missing data.

Every conclusion must be tied to observable, measurable evidence on the panel. If the evidence does not support a conclusion, do not make it.