By: Don Penven
The interpretation of blood spatter was first mentioned in a paper written in the 1890s by a researcher at the Institute for Forensic Medicine in Poland, Dr. Eduard Piotrowski. His work, "Concerning the Origin, Shape, Direction and Distribution of the Bloodstains Following Head Wounds Caused by Blows." But it took nearly 50 years before cases that included the interpretation of blood spatters appeared.
A significant number of high profile criminal cases have been prosecuted when blood spatter evidence was included among the physical evidence. One such case was the highly publicized trial of the State of Ohio v. Samuel Sheppard. In 1955, Dr. Paul Kirk offered an affidavit on blood spatter analysis, which was the earliest instance for the legal system to be made aware of the value of blood spatter analysis. Kirk’s analysis was able to show the location of the victim and the assailant, and it showed that the victim was struck by the assailant’s left hand.
Other cases involving prominent personalities where blood spatter entered into the evidence were the O.J. Simpson murder case and the trial of noted author, Michael Peterson of Durham, NC.
A leading authority in this field is Dr.Herbert MacDonell, who published in 1971, "Flight Characteristics of Human Blood and Stain Patterns." MacDonell has trained countless crime scene investigators in blood spatter analysis as well as training analysts.
The blood spatter analyst is seeking the following information from blood stain patterns:
1. Is it blood? This can be determined by simple, portable field tests using chemicals such as Phenolphthalein or Leucomalachite. These tests are presumptive in nature and are not proof positive that the substance is blood. But the likelihood is that the substance is probably blood and not tomato juice or paint. But these tests do not differentiate between human and animal blood, which require further laboratory analysis.
2. Trajectory of the blood droplets: Using visual examination at first, the analyst can tell whether a droplet struck the surface at a 90-degree angle or at some more acute angle. When a blood droplet strikes the surface at 90-degress it leaves a circular stain. If it travels into the surface from angle, the stain will be elongated—resembling an exclamation point with the narrow end of the stain pointing in the direction of travel—away from the source.
3. Using specific measurements of the actual size of the stain (length vs. width), it can determine the actual angle of travel to the surface using a trigonometric formula.
4. Once the angle of incidence is determined, the analyst attaches strings to the surface that simulate the flight path of these droplets. The strings will all cross each other at some distance from the surface and this indicates the point of origin of the various stains.
Using this method, the analyst can determine if the victim was standing or kneeling and how far away he was from the surface that contained the blood stains. For photographs that illustrate the above steps, see the article posted HERE.
To become a blood spatter analyst, specific training is required to reach the point that the analyst is accepted as an expert in this field. Additional articles on blood evidence will be posted on this site shortly. To learn more about how CSIs identify possible blood stains at the crime scene, download a free technical bulletin at the website listed below.