time of death – Lablogatory

“Those who cannot remember the past are condemned to repeat it.”

– George Santayana.

Unfortunately, there are many mistakes to learn from if we examine the history of forensic science. Despite being a relatively new discipline, there have been several disastrous failures that were only realized following the advent of DNA analysis – bite mark analysis, tool mark comparison, and arson investigation techniques (to name but a few) have all contributed to past wrongful convictions. Suffice to say that there is historical precedent for “bad” science making its way into the courtroom (see https://innocenceproject.org/misapplication-of-forensic-science/ if you’re interested in reading more).

Lately, I’ve seen several articles about a new method of determining time of death – analysing the “microbial succession” of a decomposing body.^1,2 We reviewed the basics of time of death in a previous blog https://labmedicineblog.com/2023/01/25/determining-time-of-death-separating-science-from-pseudoscience/), where we established that estimating postmortem interval is nowhere near as precise as depicted on television. This new technique hopes to change that, and the concept is ingenious. We know the body’s microbiome shifts as postmortem decomposition and putrefaction progress. By measuring and quantifying these changes in different body sites over time (using 16s rRNA sequencing), researchers then identify how our bacterial profiles change. These patterns can then be used to estimate postmortem intervals in cases where it is unknown.

Despite the impressive nature of the preliminary data, I have several reservations about the intent of this research. Many articles discuss microbiome analysis in the context of investigating homicidal deaths, and mentioning this technique in the same sentence as fingerprint and bloodstain evidence draws a direct connection in the readers’ minds to a criminal investigation. It isn’t an unreasonable jump; considering the budgetary limitations of most forensic offices, such an innovative test would likely only be performed in high-stakes cases. If we follow this chain of logic, there is a good probability that this kind of “evidence” would eventually end up as a factor in a homicide trial. When we face the risk of convicting an innocent person, sending them to death row or a life of imprisonment, our excitement around scientific achievement needs to be tempered with pragmatism.

Research environments are typically well-controlled, in stark contrast to the variety of situations in which people die. This most recent study included 36 cadavers in varying environments; the largest study to date included 63 cadavers had 63.^3,4 This sounds like a large number, but imagine the number of variables that need to be considered. Even with attempts to consider factors like soil moisture levels and temperature, the same inevitable problem will arise: each decedent will represent new, unique variables outside of our existing dataset. What if the body has been set on fire? Covered with bleach? Heavily soiled with blood, feces, or vomit? How would gastrointestinal injuries affect the microbiome? Add in varieties of body habitus, baseline commensal bacteria, and environmental variations – the possibilities are nearly endless.

Something can also be statistically significant yet lack practical utility. The reported precision of this method is highly variable between different studies. One recent study estimated time of death within +/- 3 days,³ but other studies have shown higher uncertainty (up to +/- 34 days).⁴ But how does this error rate compare to our gold standard of investigative context to determine someone’s “window of death”? When were they last seen alive? When did they last text someone, or post on social media? What’s the expiration date on the milk in their fridge? These are methods that seem less “scientific” to a layperson, but they are much more reproducible.

The researchers acknowledge the preliminary nature of these findings, and note further studies are needed. With these admissions, it may sound like my concerns are overly pessimistic. However, even if scientists and pathologists can understand the limitations and nuance, can we also expect lawyers and law enforcement professionals to understand and act accordingly? Most lawyers, judges, and police officers do not have a scientific background.

There may be occasions where a rough estimate is appropriate and helpful to an investigation. If resources eventually allow adoption of microbiome testing on a widespread, affordable basis, I’m sure many families would be interested in knowing what it means for their loved one. But the uncertainty is too high right now to accept microbiome analysis as a tool in criminal proceedings. A high level of scientific scrutiny needs to be applied before any new forensic science techniques are adopted in the courtroom. If this test could possibly be the deciding factor in a person’s innocence or guilt, we need to be absolutely certain the science behind it is quantifiable and reproducible, lest we allow mistakes of the past to be repeated.

REFERENCES:

Barron, Madeline. “Microbial fingerprinting: postmortem microbiome and forensics”. American Society for Microbiology. Published June 3, 2022. Accessed May 18, 2024. https://asm.org/articles/2022/june/microbial-fingerprinting-postmortem-microbiome-and

Schwaiger, Christopher, and LiveScience. “‘Microbiome of death’ uncovered on decomposing corpses could aid forensics”. Scientific American. Published Feb 27, 2024. Accessed May 18, 2024. https://www.scientificamerican.com/article/microbiome-of-death-uncovered-on-decomposing-corpses-could-aid-forensics/#:~:text=’Microbiome%20of%20Death’%20Uncovered%20on%20Decomposing%20Corpses%20Could%20Aid%20Forensics,-Microbes%20that%20lurk&text=The%20same%20%E2%80%9Ckey%20decomposers%E2%80%9D%20show,their%20location%20or%20surrounding%20climate.&text=Microbiology-,Microbes%20that%20lurk%20in%20decomposing%20human%20corpses%20could%20help%20forensic,death%2C%20a%20new%20study%20finds.

Burcham, Z.M., Belk, A.D., McGivern, B.B. et al. A conserved interdomain microbial network underpins cadaver decomposition despite environmental variables. Nat Microbiol 9, 595–613 (2024). https://doi.org/10.1038/s41564-023-01580-y

Tozzo P, Amico I, Delicati A, Toselli F, Caenazzo L. Post-Mortem Interval and Microbiome Analysis through 16S rRNA Analysis: A Systematic Review. Diagnostics (Basel). 2022 Oct 31;12(11):2641. doi: 10.3390/diagnostics12112641. PMID: 36359484; PMCID: PMC9689864.