Bloodstain Pattern Analysis
This domain tests whether you can classify bloodstains by the physical mechanism that produced them, extract directionality and angle from individual stains, and reconstruct a three-dimensional area of origin without overstating certainty. Questions are terminology- and geometry-heavy, and they reward examiners who separate what a pattern proves from what it merely suggests.
Build the classification first
Before naming a pattern, state three things: the mechanism (gravity, momentum transfer, expiration, arterial pressure), the direction of travel, and any post-deposition alteration. Passive patterns include drip, flow, pool, and saturation. Spatter patterns include impact, cast-off, arterial gush, expiration, and back spatter. Altered patterns include wipe, swipe, transfer, void, and skeletonization. A stain that has been wiped is evidence of cleaning, not of the original event.
Direction and angle
A tail or scalloped edge points in the direction of travel. For a clearly elliptical stain, the impact angle follows sin θ = width ÷ length: measure the ellipse only, excluding the tail and satellite droplets. Directional lines from several stains converge at the point of convergence in two dimensions; combining that distance with each impact angle gives the area of origin in three dimensions.
Worked example
A stain measures 5 mm wide and 12 mm long. sin θ = 5 ÷ 12 = 0.417, so θ ≈ 24.6°. If the stain lies 60 cm horizontally from the point of convergence, the origin height is h = 60 × tan 24.6° ≈ 27.5 cm above the stain plane. Report the uncertainty: a single stain is weak evidence, and surface texture can distort the ellipse.
Traps that cost marks
- Including the tail in the length inflates the angle.
- Applying the sine formula to irregular, overlapping, or absorbed stains.
- Calling every fine mist "high-velocity" when velocity was never measured.
- Treating the point of convergence as the area of origin.
- Ignoring substrate angle, porosity, and saturation when interpreting directionality.