Time-Since-Deposition Signatures for Canine Blood Based on Cellular Autofluorescence

Animal cruelty investigations can be hampered by not knowing when a biological sample was deposited at a crime scene, i.e., the sample's time-since-deposition (TSD). The goal of this study was to characterize a new signature system for estimating the TSD of canine blood samples based on changes in the autofluorescence spectra of cell populations that occur over time. The results showed that the intensity of cellular autofluorescence measured in two detector channels (523/30 nm and 695/50 nm; 488 nm excitation) could clearly differentiate older blood deposits (i.e., samples with a TSD of either 6 months or 1 year) from younger samples that had a TSD of less than three months. Further, cellular autofluorescence was strongly correlated with time when the sample TSD was between one day and three months, with correlation coefficients ranging between 0.83 and 0.91. A multiple linear regression model based on autofluorescence variation was tested on mock casework samples and yielded residual errors between 2 days and 35 days, with the lowest residuals observed in samples that have a TSD less than 6 days. This information can provide probative leads during an investigation of an illicit canine fighting event or an individual injury and also allow animal cruelty agencies to triage blood samples based on their TSD for downstream analyses.