Exploring the effect of specific packed cell volume upon bloodstain pattern analysis: blood drying and dry volume estimation
Bethany A. J. Larkin, Craig Edward Banks · Canadian Society of Forensic Science Journal · 2015
Bloodstain Pattern Analysis (BPA) involves the interpretation of distinct blood patterns found at crime scenes after a bloodletting incident. Much time and research has been dedicated to the quantification and analysis of blood due to its occurrence at crime scenes, establishing ways in which to map the sequence of events and general movement within the crime scene. In this paper we explore the fundamental mechanism of drying blood and the effects of packed cell volume percentage (PCV%) upon BPA investigations. Since PCV% can fluctuate between 15–75%, and in turn directly affect blood viscosity, it is important to consider its impact when evaluating bloodstains. Specific packed cell volumes (PCV%) between 15 and 75% were investigated, relating to the highest and lowest levels found in potential victims or suspects. Observations illustrated the appearance of the “coffee-ring” effect as an important mechanism of drying blood, building a thicker outer ring of red blood cells as the drop evaporates and dries, the so-called the blood ring effect. There was also found to be a significant difference in the appearance of the final bloodstains when comparing PCV%. Owing to the lower PCV% being made up of mainly plasma, bloodstains became more transparent under microscopic observation. The opposite conclusions can be made for higher PCVs where bloodstains were much thicker around the periphery of the stain. Crime scene relatable drying studies were also considered; skeletonization (relating to drying time) and volume estimation were all investigated in relation to PCV%. Three volume estimation studies were performed: dry weight, scaled photographs and spectrophotometry. It was found that all were unsuitable for volume estimation, when considering PCV% and surface type. Since PCV% cannot be established in dried blood, these volume estimation methods should not be used as the errors would be too great. Skeletonization (drying time) was not altered by PCV%.