Background/Objectives: The goal of this study was to characterize changes in autofluorescence of epithelial cells obtained from saliva stains that occur with time and investigate the potential for these changes to serve as time-since-deposition (TSD) signatures for this sample type. Methods: Saliva from 50 individuals was used to create 208 deposits that were aged between one day and nine months. Autofluorescence profiles of individual cells were obtained from each sample using imaging flow cytometry (IFC) and analyzed across nine different emission channels ranging between 435 nm and 800 nm. Results: Results showed strong evidence for linear increases in autofluorescence intensity when epithelial cells from a single donor deposit were measured over time (12 of 14 donors r ≥ 0.9). When autofluorescence profiles from all 50 donors were combined into a single time series, variation in autofluorescence intensity was observed between individual deposits with the same TSD. This inter-contributor variation decreased the overall strength of the linear relationship (r = 0.83) and yielded residual errors of ~8 days for samples that were actually 1 day old and ~82 days for samples that were over 180 days old using a linear regression model. Although this approach may not currently be amenable to estimating TSD to the day with high accuracy, clear, non-overlapping differences in autofluorescence intensity were still observed between certain time intervals, e.g., saliva deposits that were aged for 1 day compared to saliva deposits that were aged for more than 120 days. Conclusions: This suggests that cellular autofluorescence signatures have the potential to be probative when hypotheses for sample deposition involve disparate time intervals or as a screening tool for identifying which samples are most likely relevant to the crime in question based on their deposition time.
DeCorte et al. (2026) studied this question.