Distinguishing between expirated blood and impact spatter using chemical and physical methods

Abstract

In forensic investigations the distinction between spattered blood (from blows) and expirated blood (from the mouth, nose or lungs) can be vitally important. A number of cases in New Zealand and overseas involving possible expirated bloodstain patterns have been reviewed. There were many examples in overseas casework where this distinction was a central issue at trial. There is very little published literature in this area. Therefore, this project aimed to characterise expirated blood (from the mouth) using physical and chemical methods, on two surface types, namely cotton fabric (absorbent) and glossy cardboard (non-absorbent). High-speed video analysis was used to study the process of expiration. This showed that the front teeth can affect the resultant pattern; causing some stains to be separated from the main pattern, and that they are also involved in producing the beaded stain, a characteristic of expirated blood. The beaded stain is not seen in all expirated patterns. It is likely to occur on non-absorbent surfaces close to the source of the expirated blood. Air-bubbles in bloodspots were also captured in flight from the mouth. This analysis confirmed beading and air-bubbles/bubble rings as useful indicators of expirated bloodstain patterns. Alpha-amylase testing, applied soon after the event on larger bloodspots (> 3mm) in the pattern, is a valuable marker for saliva. However, in situations where only a few small bloodspots were available for testing a false negative result is possible. Therefore for items stained with only a few small bloodspots more sensitive techniques may be required. Microscopy of extracts from bloodspots stained with Haematoxylin and Eosin was used to examine cells that were present. Epithelial cells were detected in many expirated bloodspots, including small spots. This staining procedure is not specific for buccal epithelial cells. A new staining method developed by Vintiner et al [1] needs to be investigated as it can distinguish buccal cells from other epithelial cells. Oral microbial DNA analysis is also a potentially useful marker. Oral bacteria were identified from an expirated bloodstain pattern. However, further development is needed to increase the sensitivity of this method.