Evaluating the effect of body fluid mixture on the relative expression ratio of blood-specific RNA markers

The estimation of the time elapsed since a biological stain was deposited at a crime scene can provide crucial information to a forensic investigation, indicating either when a crime was committed, or whether the biological evidence was deposited at the time of a known crime event. This would enable the investigators to limit the number of suspects and to assess alibis. The relative expression ratios (RERs) of body fluid-specific RNA markers are promising molecular tools for indicating the age of biological stains. However, the nature of some forensic samples found at crime scenes could be challenging, as they frequently occur in a mixture of different body fluid types. The research presented here has utilised reverse transcription quantitative PCR (RT-qPCR) to explore the impact of bloodstains being present in mixtures with other body fluids (saliva or semen) on the resulting RERs of blood-specific markers. The expression level of three blood-specific markers (HBA, HBB and miR16) along with two reference genes (18S and U6) were analysed across multiple ageing time points in pure and mixed bloodstains. For some markers, no significant differences were found when comparing RERs in pure and mixed bloodstains, however some RERs were altered in mixed stains. This indicates that the presence of body fluid mixtures may have a significant effect on the RERs of some blood-specific markers. This should therefore be considered when selecting markers for estimating the age of stains, particularly when multiple body fluids are thought to be present.