Blood spat­ter is an impor­tant topic in the courtroom..

The open ques­tion is where is the val­i­da­tion for this dis­ci­pline of foren­sic science?

If you have ever been involved in any sort of homi­cide case, you will almost always run across the “blood whis­per­ers” which are folks who hon­estly believe that blood left at a scene can “tell us the story of the crime”.

Some thoughts:

Blood spat­ter analy­sis can be help­ful in a num­ber of dif­fer­ent ways. It can mostly be used as a way of elim­i­nat­ing pos­si­bil­i­ties. By itself, it can­not con­firm or prove what actu­ally occurred. It is at most an exclu­sion­ary tool. How­ever, as cur­rently prac­ticed in the United States today, it is being be used to sug­gest the kind of injury sus­tained, the final move­ments of the vic­tim, and the angle of the shoot­ing or attack.

One of the assump­tions made by those who prac­tice this form of foren­sic sci­ence dis­ci­pline is that in exam­in­ing the stains, the num­ber of stains found will be depen­dent on the nature and site of the injury, the pres­ence of any inter­me­di­ate tar­gets such as cloth­ing, and the nature of the pro­jec­tile used to cause the wound. Fur­ther­more, they posit that the closer the pro­jec­tile is to the tar­get, the more spat­ter will be found and such spat­ter will look like as if it was cre­ated from a spray can. Con­versely, they pos­tu­late that the far­ther away the pro­jec­tile is from the tar­get, the less mist-like spat­ter stains will be produced.

They attempt to sub­jec­tively cat­e­go­rize the uni­verse of pos­si­bil­i­ties of spat­ter into two dif­fer­ent types of spat­ter: for­ward spat­ter and back­ward spat­ter. Both types, they say, will be emit­ted in a con­i­cal fash­ion in that the impact of the pro­jec­tile will cre­ate a path which expands out from the impact site and grow in width as the spat­ter moves for­ward as it is pro­jected. Back spat­ters, they con­tend, will almost always be pos­si­ble whereas for­ward spat­ter will only occur when an exit wound occurs.

Addi­tion­ally they state that the fol­low­ing as basic fun­da­men­tal principles:

• Gun­shot spat­ter, in par­tic­u­lar, will exhibit larger stains beyond that of the aerosol or atom­ized stains dis­cussed above. These larger stains will usu­ally dis­play a 1–2 mm diam­e­ter and will be included among the aerosol/atomized stains (those which appear as if cre­ated by a spray can). Due the small size of the blood, the blood will only carry 4–5 feet. When shot at a 4 foot range, the gun­shot wound could deposit stains on the weapon, shooter’s body and or any cloth­ing involved., Fur­ther­more, the small size of such blood can also cause such stains to be eas­ily over­looked both the by the shooter and by the investigator.
• The for­ward spat­ter involved will be more sym­met­ri­cal than the back spat­ter due to the pri­mary force of the impact being trans­mit­ted in the direc­tion of the pro­jec­tile. Addi­tion­ally, for­ward spat­ter will result in a more intense pat­tern because the major­ity of the pro­jec­tile force is con­cen­trated along the vec­tor of the pro­jec­tile. The back spat­ter, on the other hand, will be less defined, slightly larger, and have fewer par­ti­cles. While back spat­ter is more pos­si­ble than for­ward spat­ter, the pres­ence of back spat­ter will be partly depen­dent on the man­ner and dis­tance at which the weapon is held.
• While a num­ber of sci­en­tists have debated what causes the spat­ter, there appears to be some agree­ment that the spat­ter is cre­ated by the col­lapse of the tem­po­rary wound cav­ity which is cre­ated by pro­jec­tile dis­rupt­ing the victim’s tis­sue. In a sit­u­a­tion where two shots are fired, the blood spat­ter from the sec­ond shot will be much heav­ier because the tis­sue sur­round­ing the first wound would com­press and cause the blood fill­ing that first wound to “squirt” out its con­tents and the closer the loca­tion of the wounds to each other, the more dra­matic the effect, result­ing in heav­ier spat­ter from the sec­ond shot.
• Finally, cast off stains will result from blood being flung from a sec­ondary object which could be from the weapon, vic­tim, or subject.

One of the many prob­lems with all of this is the true lack of mean­ing­ful val­i­da­tion of these tenets. There is more. There is also a lack of edu­ca­tion in the prac­ti­tion­ers as to the real sci­ence that involves the above. They attempt to apply and to inter­pret the real sci­ence based upon that which they objec­tively see to extrap­o­late it out to that which they sub­jec­tively con­clude with­out the basic under­stand­ing of “why”.

Fre­quently, these folks have no real under­stand­ing in fluid dynam­ics [which is a sub-discipline of fluid mechan­ics that deals with fluid flow—the nat­ural sci­ence of flu­ids (liq­uids and gases) in motion], Rhe­ol­ogy [which is the study of the flow of mat­ter: pri­mar­ily in the liq­uid state, but also as ‘soft solids’ or solids under con­di­tions in which they respond with plas­tic flow rather than deform­ing elas­ti­cally in response to an applied force], can­not explain or under­stand the dif­fer­ence between Vis­coplas­tic­ity [which is a the­ory in con­tin­uüm mechan­ics that describes the rate-dependent inelas­tic behav­ior of solids] and Vis­coelas­tic­ity [which is the prop­erty of mate­ri­als that exhibit both vis­cous and elas­tic char­ac­ter­is­tics when under­go­ing defor­ma­tion] and they can­not reduce their the­ory to any sort of math­e­mat­i­cal equa­tion involv­ing these concepts.

There cer­tainly needs to be more val­i­da­tion in this dis­ci­pline of foren­sic science.