Forensic toxicology uses analytical chemistry to detect and measure drugs, alcohol, and poisons in biological specimens after death. A toxicology report provides objective data on the chemical environment within the body at the time of or shortly after demise. The primary purpose of this report is to establish the presence or absence of chemical substances that may have contributed to, caused, or complicated the death. This analysis gives medical examiners and forensic pathologists valuable evidence to understand the circumstances surrounding the passing.
Collecting Samples for Analysis
The initial phase of post-mortem analysis involves collecting multiple biological specimens during the autopsy. Peripheral blood, often drawn from a limb vein, is considered the most representative sample. It is less affected by post-mortem changes and best reflects the concentration of substances circulating just before death. Central blood, taken directly from the heart, is also collected but is more susceptible to chemical changes after circulation stops.
Vitreous humor, the fluid within the eyeball, is frequently analyzed because it is isolated from decomposition effects. Substances like alcohol or specific drugs remain stable in this fluid, providing a useful secondary comparison point to blood concentrations. Solid tissues, particularly the liver, are also analyzed because the liver is the primary site for drug metabolism and can accumulate certain drugs or heavy metals.
The contents of the stomach are examined to determine if a drug or poison was recently ingested, offering insight into the timing and quantity of exposure. Urine is often collected for screening, as it contains drug metabolites indicating recent exposure, even if the parent drug has cleared the bloodstream. For investigations into chronic exposure, hair and nail samples provide a historical record of sustained drug use over weeks or months.
Categories of Compounds Tested
Forensic laboratories conduct broad screening tests designed to detect chemical agents that could influence the cause of death. One major category includes therapeutic drugs, encompassing prescribed medications and over-the-counter substances like pain relievers. These screenings look for common compounds to understand existing medical conditions or potential misuse, including:
- Opioids
- Benzodiazepines
- Antidepressants
- Anticonvulsants
Another focus is on drugs of abuse, including illicit substances such as:
- Cocaine
- Methamphetamine
- Heroin metabolites (such as morphine)
- Cannabis compounds
Analytical methods detect the parent drug and its metabolites, confirming the substance was processed by the body. Volatile substances are also a priority, primarily ethanol (alcohol), but also compounds like methanol, isopropyl alcohol, and inhalants.
Poisons and heavy metals constitute a third category, requiring specialized testing for compounds like arsenic, lead, or mercury. These substances are not part of routine drug panels and often require techniques like mass spectrometry for identification and quantification. A separate test is performed for common gases, most notably carbon monoxide, which binds tightly to hemoglobin and is a frequent cause of death.
Understanding Post-Mortem Results
Interpreting numerical concentration results is complex, requiring more than simply identifying a substance’s presence. Toxicologists must distinguish between a therapeutic concentration (the expected level for a prescribed dose) and a toxic or lethal concentration. A toxic level indicates a concentration high enough to cause adverse effects, while a lethal concentration is statistically associated with causing death.
Finding a drug does not automatically signify the cause of death; a therapeutic level of medication may be present without being the fatal agent. The measured concentration must be correlated with clinical data and established toxicological reference ranges to determine its potential impact. These reference ranges are derived from research and case studies, offering a guide for interpretation.
A major hurdle in post-mortem analysis is Post-Mortem Redistribution (PMR), which affects reported drug levels. PMR describes the movement of certain drugs from high-concentration storage sites (like the liver or lungs) back into the central blood after circulation stops. This movement can artificially inflate the drug concentration in central blood samples, making the level appear higher than it was before death.
To address PMR, toxicologists compare drug levels in central blood to those found in peripheral blood or vitreous humor, which are less affected. Decomposition, or putrefaction, presents another challenge by potentially altering drug concentrations through bacterial activity. Bacteria can sometimes generate ethanol within tissues, or conversely, destroy or metabolize certain drug compounds. Toxicologists must apply specialized knowledge and correction factors to ensure the reported results accurately reflect the chemical state before death.
How Toxicology Informs Cause of Death
The final toxicology report is delivered to the medical examiner or forensic pathologist. They integrate the chemical findings with physical evidence from the autopsy and the circumstances of the investigation. The toxicological data helps determine the specific mechanism of death, which describes the physiological derangement that led to the cessation of life. For example, a high opioid concentration may establish the mechanism as respiratory depression induced by drug intoxication.
This determination is used to classify the manner of death, which is the official categorization of the circumstances surrounding the fatality. The manner of death is designated as one of five categories:
- Natural
- Accident
- Suicide
- Homicide
- Undetermined
Finding a lethal concentration of a prescription drug, combined with scene evidence, might lead to a classification of suicide or accidental overdose, depending on the context.
If the report identifies a substance like carbon monoxide at lethal levels, the pathologist must decide if the manner was accidental (such as a faulty furnace) or intentional. The toxicological results provide the objective chemical evidence necessary to support the pathologist’s final conclusion. This integration ensures the determination of both the mechanism and the ultimate manner of death is scientifically supported.