Abstract
In forensic investigations the distinction between expirated bloodstains (blood from the mouth, nose or lungs) and impact spatter (blood from gunshots, explosives, blunt force trauma and/or machinery accidents) is often important but difficult to determine due to their high degree of size similarity, which may result in the patterns being incorrectly classified. Expirated bloodstains on an accused person’s clothing occur when assisting an injured person, a finding which would tend to exonerate that individual. Impact spatter stains on clothing tend to occur due to the proximity of the person to the bloodshedding event, implying guilt. Therefore this project determined the characteristics inherent in each bloodstain type by using high speed digital video analysis and developed a test using PCR analysis to distinguish between the two types of bloodstain patterns to allow for proper bloodstain classification.
The current study developed a test involving PCR analysis using DNA from human-specific oral microbes as a biomarker for the presence of saliva and hence oral expirated bloodstains. This PCR method is very specific to human oral Streptococci, with no PCR product being made from human DNA or DNA from soil or other microbes that were tested. It is also very sensitive, detecting as little as 60 fg of target DNA. The PCR was not inhibited by the presence of blood and could detect target DNA in expirated blood for at least 92 days after deposit on cardboard or cotton fabric. In a blind trial, the PCR method was able to distinguish three mock forensic samples that contained expirated blood from four that did not. These data show that bacteria present in the oral cavity can be detected in bloodstains that contain saliva, and therefore can potentially be used as a marker in forensic work to distinguish mouth-expirated bloodstains from other types of bloodstains.
High speed digital video analysis was used to examine the physical characteristics, droplet sizes and distance travelled of expirated bloodstain patterns generated by breathing, spitting and coughing. This analysis showed that some physical characteristics (beaded stains) are unique to expirated bloodstains and can be used to determine that the stain is expirated in origin. However, the video images also showed that these physical characteristics are not always present and that the type of expelling mechanism i.e. coughing, spitting or breathing, the distance the bloodstains travel, and the type of surface the bloodstains are deposited on all influence the presence of these physical characteristics.