Forensic science is responsible for providing scientific evidence for use in courts of law. One of the most common, and arguably most important, procedures performed by forensic scientists is the time of death, or the post-mortem interval (PMI) estimation.
Analysis of the physical changes of the body, including body temperature, muscle stiffness, and hypostasis, provides accurate and reliable PMI estimation up to several hours since the death. After this period, the technique becomes more prone to error and other types of analysis are used, such as DNA degradation and forensic entomology. However, these methods are most reliable from weeks to years after death. Therefore, there is a window from several hours to several weeks where PMI estimation lacks accuracy.
One promising technique to fill this gap is metabolomic profiling. After death, cells begin to lyse, releasing their components and leading to an increase in metabolite concentration in bodily fluids, such as blood. If there are areas where these changes occur linearly, and for a relatively long period of time, accurate and reliable data could be obtained for PMI estimation.
A recent study from the International Tomography Center and Novosibirsk Regional Clinical Bureau of Forensic Medicine, Russia, has been able to identify several promising metabolites for use as biomarkers in PMI estimation.
Samples of blood, aqueous humor (AH), and vitreous humor (VH) were taken from human cadavers at several different timepoints. These samples were then processed to remove any proteins or lipids that may affect the results, and then analyzed nuclear magnetic resonance (NMR) using a Bruker Biospin Avance III HD 700 MHz NMR spectrometer.
Six out of the 42 metabolites identified in all three samples demonstrated strong positive linear correlation with PMI: hypoxanthine, choline, creatine, betaine, glutamate, and glycine. However, as the metabolites in the blood sample varied more chaotically than the AH and VH samples, it was concluded that this bodily fluid was not suitable for PMI estimation.
Metabolomic analysis of the ocular fluids, AH and VH both provided accurate and reliable PMI assessment. However, the VH had the strongest positive linear correlation between PMI and metabolite concentration, probably because the VH has a higher viscosity and is more isolated from the vascular system than the AH. Although the AH does have some advantages over the VH: there is less risk of blood contamination when sampling and sample preparation is more straightforward.
As the study identified multiple metabolites with a positive linear correlation to PMI, the researchers also performed PMI estimation with 2–3 biomarkers for both ocular fluids. This combination analysis was able to significantly reduce the errors in the PMI correlation, increasing the accuracy and reliability of PMI estimation.
This study demonstrated that metabolic profiling shows great promise to fill the gap in PMI estimation between several hours and several weeks. The most accurate and reliable data is obtained from anatomically isolated bodily fluids, such as ocular fluids, and using a combination of metabolite biomarkers.
- Zelentsova, E., A., et al. (2020). Post-Mortem Changes in Metabolomic Profiles of Human Serum, Aqueous Humor and Vitreous Humor. Metabolomics. https://doi.org/10.1007/s11306-020-01700-3.