It is when external bleeding occurs, that bloodstain pattern analysis can be used to interpret the events at a scene.

Bloodstain Pattern Analysis can be helpful in interpreting not just homicide scenes but any scene where bloodletting has occurred.  Bloodstains can be studied on a variety of surfaces including walls, floors, and ceilings inside structures, vehicles, as well as on sidewalks, driveways and even soil.

WHAT BLOODSTAIN EVIDENCE CAN REVEAL:
A bloodstain pattern analyst uses math and physics coupled with casework and experience to interpret bloodstains at a crime scene, and help to reveal the:

• Area(s) of Origin(s) of the blood.
• Distance between the target surface and the origin at the time of the incident.
• Type and direction of impact.
• Number of blood-letting incidents (blows, shots, etc.)
• Position of victim during the attack. Movement and direction of suspect and victim during and after bloodshed.

PASSIVE BLOOD:  Is blood primarily acted on by gravity and includes:

• Clots,
• Drip patterns, and drops
• Flow patterns,
• Pools of blood.

PROJECTED BLOOD:  Is blood which is exposed to a force or energy causing the blood to be dispatched.  Projected blood Includes:

• Arterial Spurt Patterns,
• Cast-Off Stains,
• Medium Velocity Impact Spatter Patterns,
• Satellite or Secondary Impact Stains,
• Back Spatter,
• Forward Spatter, and
• High Velocity Impact Spatter Patterns.

NON-PROJECTED BLOOD:  May best be described as Transferred Bloodstains.  A transfer stain occurs when a bloody object comes in contact with another (often clean) surface or object or a bloody object comes in contact with a clean surface.  The resulting pattern is a bloody impression on the target surface.  The interpretive value of this type of pattern includes:

• The identification of an object or a person.
• The direction or movement of an object or a person.
• The sequencing of events at a scene.

BLOODSTAIN CHARACTERISTICS (Stain Size):
Size and edge character are far more dependent upon the texture of the target surface than on the distance fallen.  The size of a stain left by one drop of fallen blood varies according to the blood volume, distance fallen, and the target surface texture.  It is rare to have knowledge of blood volume from a crime scene drop, therefore estimates of stain size versus height fallen would not be based on scientific study.

BLOODSTAIN CHARACTERISTICS (Stain Shape):
Surface texture is the key characteristic in interpreting the bloodstain evidence.  In recreating the bloodstains, identical surfaces are necessary to validate the reconstruction. 
More porous surfaces (like raw lumber, papers and concrete) create spines and scalloped edge characteristics.
Smooth, hard and less porous surfaces (like glass and glazed tile) tend to produce smoother edge characteristics with fewer spatters noted.

TARGET SURFACE CHARACTERISTICS and SPATTER:
A single drop of blood has considerable flexibility due to surface tension and acts much like a water balloon.  Upon impact with a non-porous surface, the blood drop spreads out resulting in a concise scalloped edge.  When a drop of blood impacts a porous surface, the protuberances rupture the surface tension, and the contents “spill” out in all directions resulting in an atypical or asymmetrical bloodstain.

DIRECTIONALITY OF BLOODSTAINS:
Determining the directionality of blood droplets is possible because a droplet striking an angled surface produces a teardrop-shaped bloodstain.  The pointed end, “points” in the direction the blood traveled.

The “teardrop-shaped” stain is affected by the physical law of inertia:  The resistance of a moving body to any force operating to change its motion, direction or speed.  Therefore, as the speed is dissipated abruptly by the surface upon which it impacts, the blood volume is consumed, and the droplet trails off to create a pointed end.  The greater the angle of impact, the more elongated and narrower the stain will appear.  Primary blood droplets may produce smaller cast-off spatters which point back to the main stain.  This is the only time blood will not “point” in its direction of travel.

IMPACT ANGLE:
Vertical drop strikes an angular surface, or a blood drop falling onto a flat surface that is not horizontal will produce an elliptical rather than a circular stain.
As the angle becomes more acute (i.e., 10 degrees) the stain becomes more elongated.
For the most part, the greater the angle of occurrence, the more elongated the resulting stain.
From the elongated stains, ratios can be calculated to establish impact angles.

ANALYSIS OF BLOODSTAIN PATTERNS:
The initial step in determining the origin of the blood involves selecting a number of bloodstains from the sample, then measuring their length and width to determine the length/with ratio.

The bloodstain analyst will then draw a line through the long axis of multiple bloodstains, backwards to where the rays converge.  Where those rays converge is referred to as the Area of Convergence.

Once the Area of Convergence is established, and the bloodstain’s mathematical value obtained, the bloodstain analyst then converts the length/width ratio into the Angle of Impact, and the Area of Origin can be calculated for each individual stain.

BLOOD DRIPPED INTO BLOOD:
As blood drips into a static pool of blood, many small droplets are projected into the air, they reach their peak and gravity pulls them back onto the target surface.  The resulting stain will produce relatively round, symmetrical looking stains that may be described as near 90º stains.

SPLASHED BLOOD:
Blood volume of 1.0 ml. or greater
Subjected to a minor impact or in the absence of impact, the blood falls at least four inches.
Produces long narrow streaks with very small round stains.

PROJECTED BLOOD:
Blood volume of 1.0 ml. or greater, projected downward in excess of the normal gravitational pull.  The resulting bloodstain produces a spiny pattern around the perimeter of the main stain.  An example of this pattern includes a shod foot stomped in a pool of blood.

CAST-OFF BLOOD:
Cast- off bloodstains occurs primarily when blood is flung from a blood soaked extremity, or weapon.  The pattern results in blood drops smaller than .05 ml. and the resulting stain diameter will measure .25 inch or less.  This pattern is linear (line-like) in appearance so much so that you could almost “connect the dots” in some cases.

Using cast-off stains, a bloodstain pattern analyst may be able to determine a minimum number of actions (blows, stabs, kicks).  The first event will result in little or no bloodspatter - for instance, two cast-off patterns on surface will represent a minimum of three separate actions.

When a person raises a weapon over their head, in one continuous motion, approaching a peak back swing rapid deceleration occurs, causing blood to be “cast-off” or flung from the weapon.

Both angular and 90º bloodstains are created in the cast-off stain.  The 90º bloodstains can help place the suspect in the scene.  There is often little or no blood cast-off on the forward stroke, after backstroke reversed.  The rapid termination of a weapon is the key to cast-off blood.
Directionality can be determined by examining cast-off bloodstains.  This information allows for an accurate estimation of the position of the person when the blows were struck and determination of a suspect’s right or left overhand swings.

PROJECTED BLOOD CONSISTENT WITH BLUNT FORCE TRAUMA:
A strong blunt force object impacts an exposed, bloody area of a victim (club, hammer, brick, hand or foot) strikes a lacerated scalp.

Projected bloodstains are characterized by size, shape and distribution.  A medium velocity force impacts static blood at approximately 25’ per second.

A beating typically impact spatters or what was historically referred to as Medium Velocity Impact Spatters.

This bloodstain pattern, created by a hammer strike, is representative of a medium velocity impact spatter pattern.

PROJECTED BLOOD CONSISTENT WITH HIGH ENERGY TRAUMA:
This pattern is classically associated with gunshot wounds and high RPM machinery, and explosions.  This “High Energy” produces misted or atomized blood resulting in a “mist-like” dispersion similar to an aerosol spray.  Because of the relative density of these small blood droplets, they will only travel a short distance (most researchers have found this class of blood only travels three to four feet.)  Larger bloodstains (usually less than 3 mm in diameter) will be interspersed within the misted pattern, and unlike the mist-like bloodstains, may be projected several feet.

This bloodstain pattern, resulted from a shooting, is representative of projected blood consistent with high energy trauma, and is also referred to as high velocity spatter.

NON-PROJECTED BLOOD (Transfer and Impression Patterns):
This class of bloodstains includes:

• Bloody fingerprints, hand prints and foot prints,
• Weapon transfers,
• Usually not distinct enough to effect positive identification but class characteristics may often be excellent evidence.

MOVEMENT OF VICTIM(S) AND / OR SUSPECT(S):
Swipe:  Blood soaked hand, clothing or hair are only a few ways that blood can be transferred onto a clean surface not already contaminated with blood.  The pattern usually starts with a dense area of blood then terminates with a feathered edge which helps establish movement and direction.

Wipe:  This bloodstain pattern is created when an object moves through an existing bloodstain, removing blood from the original stain and altering the appearance of the original stain.