Postmortem Metabolomics Reveals Agonal Death Processes in Opioid Overdoses and Hangings

A Chemical “Fingerprint” in Blood Could Help Forensic Experts Distinguish Opioid Deaths from Hangings

When a person dies after an opioid overdose, the body may offer forensic investigators few clear answers. Autopsies can reveal congestion in the lungs or frothy fluid in the airways, but such findings are inconsistent and rarely establish the cause of death on their own. Toxicology can confirm that an opioid was present, yet the result may still be difficult to interpret because tolerance varies dramatically between individuals and because postmortem drug concentrations do not correspond to a single, universally fatal threshold. A new study suggests that the answer may lie not in one drug or one organ, but in hundreds of small molecules circulating in the blood at the time of death.

Researchers in Sweden have used postmortem metabolomics to identify chemical patterns that distinguished deaths attributed to opioid intoxication from deaths caused by hanging. The approach correctly classified 78.2 percent of cases in a validation dataset, offering a potential new tool for difficult death investigations. The findings, published in the International Journal of Legal Medicine, do not establish a definitive diagnostic test, and the researchers stress that larger studies are needed. But they provide evidence that the body’s final biochemical state—the so-called agonal phase—may preserve clues about how death unfolded even when traditional forensic evidence is ambiguous.

Metabolomics is the large-scale measurement of metabolites, the small molecules produced or transformed by biochemical reactions in cells and tissues. These molecules include amino acids, lipids, energy intermediates and products of cellular stress. Unlike conventional toxicology, which searches for known drugs and their breakdown products, metabolomics examines a broad chemical landscape without restricting the analysis to a predetermined target. In this study, the investigators mined data already produced during routine toxicological screening of femoral blood using high-resolution mass spectrometry, a technique that measures molecules according to their mass-to-charge ratios with great precision.

The team examined 625 forensic autopsy cases registered by Sweden’s National Board of Forensic Medicine during 2021. The group included 287 deaths classified as opioid intoxications and 338 deaths classified as hangings. The cases were divided into a training set containing three-quarters of the data and a validation set containing the remaining quarter. The researchers then applied orthogonal partial least squares discriminant analysis, or OPLS-DA, a multivariate statistical method that searches for combinations of molecular features capable of separating predefined groups.

The initial dataset contained up to 1,000 molecular features, but many were associated with demographic differences rather than the cause of death. The researchers therefore removed features linked to age, sex, body-mass index and postmortem interval, reducing the potential for the model to classify people based on background characteristics. They then selected 269 features with the strongest statistical contribution to the separation between opioid intoxications and hangings. In the validation set, the resulting model correctly identified 83.5 percent of hanging deaths and 71.8 percent of opioid intoxications, producing an area under the receiver operating characteristic curve of 0.85. That performance indicates meaningful separation, although it falls short of the certainty required to replace conventional forensic investigation.

Several groups of metabolites appeared to contribute to the distinction. Opioid intoxication cases showed lower levels of isobutyryl-L-carnitine and hexanoylcarnitine, members of the acylcarnitine family. Acylcarnitines are intermediates formed as fatty acids are transported into mitochondria and broken down through beta-oxidation, a process that helps generate energy. Because mitochondrial energy production depends on oxygen and efficient electron transfer, changes in these molecules may reflect altered cellular respiration during the final stages of life. The pattern is consistent with earlier work by members of the research group, which identified reduced acylcarnitines in fatal oxycodone intoxications.

The opioid group also had higher levels of two putatively identified metabolites of nicotinamide adenine dinucleotide, or NAD, known as 2PY and 4PY. NAD is central to redox chemistry: it shuttles electrons generated during the metabolism of nutrients toward the mitochondrial electron transport chain, where cells produce adenosine triphosphate. A shift in NAD-related metabolites could therefore signal severe disruption of energy metabolism or oxygen utilization. The researchers caution, however, that most of the molecular features could not be identified with the highest level of chemical certainty. The acylcarnitines were identified using an in-house reference database, but many other compounds were only putatively annotated by matching their mass-to-charge ratios to public databases.

One possible explanation for the chemical separation is that opioid deaths and hangings often involve different timelines of respiratory failure. Opioids primarily suppress breathing through activation of μ-opioid receptors in brain-stem respiratory centers. In some cases, breathing may become progressively slower and shallower over tens of minutes or several hours before death. Hanging can also cause oxygen deprivation, but the terminal process may be considerably shorter in many cases. A prolonged period of inadequate breathing could leave a different metabolic imprint from a rapid loss of oxygen. The study cannot prove that this timing difference caused the molecular pattern, but the hypothesis gives investigators a biological framework for understanding why two forms of fatal oxygen deprivation might not look identical in blood.

The model’s errors also revealed how closely cause of death can be entangled with manner of death and underlying health. Thirteen of the 20 misclassified opioid intoxications were suicides, a statistically unusual concentration compared with the full opioid group. The intoxication cases that most strongly resembled hangings often involved evidence of seizures, very recent opioid administration or ingestion of an exceptionally large amount of the drug—circumstances suggesting a rapid agonal phase rather than prolonged respiratory suppression. Most hanging cases were suicides, whereas opioid deaths included a mixture of accidents, suicides and other classifications. That imbalance may have introduced metabolic differences related to depression, stress or other conditions rather than the physical mechanism of death itself.

The researchers attempted to determine whether the metabolome could also estimate the severity of opioid-related respiratory failure. They divided cases into four presumed stages, beginning with hangings without detected opioids and progressing through hangings with opioids, opioid intoxications without frothy airway fluid and opioid intoxications with frothy fluid. The final category was intended to represent the most severe or prolonged respiratory failure. Neither OPLS-DA classification nor partial least squares regression produced a statistically meaningful separation among these groups. The failure may reflect the weakness of frothy airway fluid as a retrospective marker: the feature was recorded only when a pathologist mentioned it, its quantity was not measured and the finding can vary widely between cases.

The study’s implications are therefore promising but deliberately limited. A metabolic fingerprint could become an additional layer of evidence when autopsy findings, toxicology and circumstances point in different directions. In a case involving both a ligature mark and a substantial opioid concentration, for example, a validated metabolomic signature might help investigators judge whether intoxication or hanging played the dominant role. It could also eventually help distinguish sudden opioid-related collapse from deaths preceded by a longer period of respiratory depression. Yet metabolomics is not immune to confounding, and the current study involved cases from a single country and one year. The researchers say future work should include larger, more balanced cohorts, better characterization of manner of death, stronger chemical identification and more direct physiological measures of respiratory failure.

For now, the findings offer a glimpse of a forensic future in which death investigations do not rely solely on visible injuries or the concentration of a drug in blood. Instead, investigators may read the coordinated changes left across pathways governing mitochondrial fuel use, NAD metabolism, amino-acid turnover and cellular stress. Such a test would not replace the judgment of forensic pathologists, nor would it provide a simple molecular answer in every case. But by capturing the body’s final biochemical response, postmortem metabolomics could give science a new way to reconstruct the last minutes or hours of life—turning an apparently chaotic chemical mixture into a more informative record of how death occurred.