Bloodstain pattern analysis examines stain location, shape and distribution to assess supported explanations. Surface properties, movement, blood characteristics and later disturbance affect the observed pattern.
Use appropriately selected observations and explicit uncertainty. Pattern size does not independently measure impact energy, weapon type, blow count, survival time or an offender’s identity.
By the end of this topic you will be able to:
- Describe passive, transfer and impact-related observations.
- Explain geometric assumptions in angle and origin estimation.
- Evaluate alternative explanations and uncertainty.
- Non-Newtonian fluid
- Shear-dependent fluid behaviour; not a unique explanation of cast-off morphology.
- Surface tension
- A surface property affected by conditions; drops need not remain perfect spheres.
- Area of convergence
- A region of projected directional convergence, not a guaranteed point.
- Area of origin
- A possible spatial region assessed with assumptions and uncertainty.
- Parent stain
- A principal deposit and associated features requiring surface and mechanism context.
- Cast-off
- Blood released from a moving object; not an exact blow-count formula.
Blood Physics: Why a Drop Behaves the Way It Does
Blood can show shear-dependent behaviour, and drops can oscillate or deform. Do not assume a perfectly spherical flight or one universal physical constant explains every stain.
Surface tension, viscosity and interaction with the target affect stain formation. A falling drop can deform and break up; the resulting stain depends on impact and surface conditions as well as the blood’s motion. Stain appearance alone is not a direct force meter.
Passive Stains: Drops, Drips, Pools and Flows
Gravity-driven drops, flows and pools depend on volume, motion, surface and environment. Stain diameter or pool dryness alone does not establish drop height or injury time.
Passive patterns can include drops, drips, pools and flows associated with gravity. Document distribution, surface slope and interruptions. A flow records movement of liquid across the surface; it does not automatically show the position of every participant.
Transfer Stains: Wipes, Swipes and Contact Patterns
Contact and movement can produce transfer patterns. A wipe-like pattern does not independently establish deliberate cleanup or the responsible person.
A contact transfer can reproduce some features of a blood-bearing surface. A swipe can deposit blood as that surface moves; a wipe disturbs an existing wet deposit. Interpret direction or sequence only where suitable features survive and credible alternatives have been considered.
Impact Spatter and the Velocity Question
Several mechanisms can produce overlapping stain sizes. Avoid assigning a unique weapon or low/medium/high impact velocity from diameter alone.
Cast-off arrangements may inform possible motion, but line count plus one is not a reliable exact blow count.
Expirated Bloodstains and Their Interpretation
Consider the basis and alternatives for a proposed expirated origin. Vacuolation, colour or height alone is not diagnostic, and a suspected pattern does not provide a precise breathing or survival interval.
Angle of Impact and Area-of-Origin Estimation
For a suitable elliptical stain under the relevant assumptions, alpha = arcsin(width/length) estimates the impact angle to the surface. Substrate and measurement quality matter.
Projected directions can constrain an area of convergence and a possible origin region. Report uncertainty; a string or software output is not a guaranteed point in space.
For an appropriately selected elliptical stain, the conventional geometric estimate uses the arcsine of width divided by length, excluding unsuitable peripheral features. The estimate depends on stain selection, target and projection assumptions. Back-projecting several stains also requires uncertainty assessment and attention to gravity and motion.
Check Your Understanding
Sources
- 1.Bloodstain pattern analysis guidance · Forensic Science Regulator
- 2.Research on bloodstain interpretation performance · National Institute of Justice