Introduction to Bloodstain Pattern Recognition

Forensic serology at a bloody crime scene is concerned with three questions: is this stain blood, is the blood human, and who did it come from. The first two are resolved through chemical and immunological tests. The third is answered by DNA typing. None of those questions requires the analyst to know whether the blood dripped, spattered, or pooled. The serology function is about identity, not dynamics.

Bloodstain pattern analysis, by contrast, is entirely about dynamics. A trained BPA specialist looks at the trajectory angles of individual spatter drops, the arc of cast-off arcs, the directionality of drip trails, and the geometry of impact origins to reconstruct an event. They may determine that a person was struck at least three times, that they were moving toward a doorway, and that the final blow was delivered while they were already on the floor. The BPA specialist does not need to confirm the stain is blood at all; that is the serologist's job and it is done before the pattern is destroyed by chemistry.

When blood falls freely from a stationary or slowly moving source, the drop forms a sphere in the air due to surface tension. On impact, it produces a stain whose shape reflects the drop velocity, the angle of incidence, and the texture of the target surface. On a smooth, hard, horizontal surface, a drop falling from below roughly one metre produces a roughly circular stain about 10-15 mm in diameter with a smooth edge. From a greater height, the edge becomes crenulated as secondary drops are thrown outward, forming what is often called a crown or satellite effect.

• Low-height drip (under 1 m): smooth circular stain, minimal edge distortion, few or no satellites.
• Medium-height drip (1-2 m): larger stain, scalloped or wavy edge, small satellite drops close to the main stain.
• High-height drip (over 2 m): pronounced crenulation, larger satellite splash rings, sometimes radial striations.
• Angled impact: the stain becomes oval, with a tail or satellite distribution on the downrange side indicating travel direction.

Passive drips are important to the serologist as orientation markers. A trail of passive drips tells you a person was standing or moving while losing blood slowly. An isolated pool with no drip trail suggests the person was still. Recognising the pattern type allows the serologist to choose the most informative stains for sampling and to advise the BPA specialist on what pre-chemical measurements are needed.

Transfer stains form when a blood-wet surface contacts another surface and deposits material onto it. Unlike projected or drip stains, transfer stains can preserve the geometric pattern of the source. A shoe sole leaves a tread impression. A bloody hand pressed to a wall leaves finger-and-palm geometry. A fabric pressed against a surface leaves a weave or texture impression. These patterns have both pattern-comparison and serological value: the serologist confirms the stain is blood and can sample it for DNA, while a pattern expert may compare the imprinted geometry against a suspected source.

A critical distinction for serologists is between a swipe and a wipe. A swipe is produced by a blood-bearing object moving across a surface, leaving a deposit that thins in the direction of travel. A wipe is produced when a relatively clean object moves through an existing bloodstain, creating a cleared track. Both produce directional information, but they encode it differently. Misclassifying one for the other produces an inverted directional conclusion.

Projected patterns are produced when blood is given momentum beyond that of gravity alone. Arterial spurting from a severed vessel produces arcing streams of parallel arcs that repeat with each heartbeat until pressure drops. Impact spatter is created when a bloodied surface is struck, expelling drops that travel outward in a cone from the point of impact. Cast-off occurs when a weapon that has accumulated blood is swung, throwing droplets in an arc that maps the weapon's path through the air.

For the serologist, the key practical point about projected patterns is that the fine drops of impact spatter dry very quickly and are easily disturbed. They should be among the first stains documented, with dimensions and positions measured, before any foot traffic near the area. They are also typically small, sometimes under two millimetres across, which limits DNA yield. A serologist should assess whether a grouped presumptive test or an individual stain sample approach is appropriate for a given density of spatter.

The cardinal rule of bloodstain scene work is that chemical testing destroys morphology. Luminol, leucomalachite green, phenolphthalein (Kastle-Meyer), and similar presumptive reagents disrupt stain edges, alter colour, and in the case of luminol, wash out fine detail entirely. Once applied, the pre-chemical pattern is unrecoverable.

1. Overall scene photography
   Wide-angle stills and, where resources allow, videography of the whole scene before any approach. Establish spatial relationships between stain locations.
2. Area-level documentation
   Mid-distance photographs of each distinct area of activity, with a scale bar and north arrow, referencing the scene plan.
3. Stain-level photography
   Close-up photographs of individual stains or patterns with a metric scale. Capture directionality, edge morphology, and satellite distribution. Note whether a right-angle scale is positioned parallel to the stain axis.
4. Measurement and mapping
   Measure stain dimensions, record positions on the scene sketch using consistent coordinates. For spatter, record the distribution perimeter before any disturbance.
5. Chemical testing
   Only after all the above is complete and verified. Choose test location so that the most informative sections of the pattern are preserved for the BPA specialist to examine if needed.

Luminol is a special case worth explicit mention. Because it can reveal bloodstains invisible under normal light, including stains that have been cleaned up, it is tempting to apply it early. The correct approach is to document everything visible first, then use luminol only after visible-light examination is complete. Luminol results should be photographed immediately with the appropriate exposure settings, because the chemiluminescence fades in minutes.

In well-resourced investigations, the serologist and the BPA specialist communicate before, during, and after scene processing. The serologist tells the BPA specialist which stains have been sampled and what chemical treatment was applied. The BPA specialist tells the serologist which areas of the pattern are most significant for pattern-interpretation purposes, so that sampling is targeted to avoid them first.

The reports produced by the two functions must be coherent. If the serology report identifies human blood from a male donor in a cluster of stains in room A, and the BPA report describes a medium-velocity impact pattern in the same room, the combined account should not contain contradictions. Inconsistencies, such as serologically confirmed blood in stains the BPA report describes as non-spatter, will be exploited in cross-examination.