Early Postmortem Changes in NADH Fluorescence Lifetime Parameters in Rat Skeletal Muscle: Potential for Postmortem Interval Estimation

Investigation of the molecular mechanisms of postmortem processes to identify postmortem interval (PMI) markers is highly relevant in forensic medicine, given the insufficient accuracy and substantial limitations of current routine PMI estimation methods. This exploratory study aimed to describe changes in NADH fluorescence decay parameters in rat skeletal muscle during the early postmortem period and to assess their potential for PMI estimation. Using time-resolved fluorescence spectroscopy, the short- and long-lifetime components (τ1 and τ2), the amplitude-weighted mean fluorescence lifetime (τm), the relative contributions of the short- and long-lifetime components (α1 and α2, respectively) and fluorescence intensity were measured in 10 Wistar rats before death, immediately after death, every 30 min during the first 6 h, and at 24 h postmortem. Immediately after death, τ1 decreased compared with the antemortem value (p = 0.02). τ1, τm, α1, and α2 were dependent on the postmortem interval. For PMI prediction within the first 6 h after death, a partial least squares regression model combining α1, τm, and the difference between rectal and room temperatures was constructed. The model demonstrated promising predictive performance, with a root mean square error (RMSE) of 0.89 h (95% CI: 0.63–0.99), a mean absolute error (MAE) of 0.68 h (95% CI: 0.49–0.82) and an R2 of 0.762 (95% CI: 0.689–0.886); the prediction error did not exceed 1 h in 73.2% of cases. These data are preliminary and require confirmation in studies with larger samples under different ambient temperature conditions and subsequent validation on human cadaveric material.