A Toxic Relationship: Forensic Testing in Decomposed Bodies
A toxicological analysis of decomposed remains presents significant challenges due to the chemical and biological changes that occur after death. These processes can create interfering substances, cause the loss of drugs or toxins (analytes), and complicate the interpretation of results.
Decomposition Products and Interferences
During decomposition, bacteria and endogenous enzymes break down tissues, producing a variety of chemical compounds. These byproducts can interfere with toxicological screening and confirmation tests, leading to false positives or inaccurate quantifications.
- Endogenous Ethanol: One of the most common issues is the production of ethanol by microbes (neo-formation). Bacteria and yeast can ferment carbohydrates in the body, producing alcohol. This makes it difficult to determine if the deceased consumed alcohol before death or if the ethanol was produced postmortem. Levels below 0.05 g/dL are often attributed to decomposition, but higher concentrations can also occur.
- Volatile Substances: Putrefaction generates numerous volatile substances, including alcohols (propanol, butanol), aldehydes, and ketones. These can interfere with the analysis of volatile poisons or drugs of abuse like ethanol.
- Ammonia and Biogenic Amines: The breakdown of proteins produces high levels of ammonia and biogenic amines (e.g., putrescine, cadaverine). These compounds can alter the pH of samples, affecting drug extraction efficiency and stability. They can also interfere with certain types of immunoassays used for drug screening.
National Library of Medicine - "Postmortem Production of Ethanol and Factors that Influence Interpretation": This article discusses the microbial production of ethanol and other volatiles that can confound toxicological analysis.
URL: https://journals.lww.com/amjforensicmedicine/fulltext/1996/03000/postmortem_production_of_ethanol_and_factors_that.2.aspx
Loss of Analyte Due to Decomposition
The stability of drugs and their metabolites is significantly compromised in a decomposing body. This loss can occur through several mechanisms, leading to false-negative results or an underestimation of the drug concentration at the time of death.
- Microbial Degradation: The same microorganisms that decompose tissues can also metabolize drugs present in the body. For example, bacteria can degrade cocaine into its inactive metabolite, benzoylecgonine, and then further break it down, potentially eliminating evidence of cocaine use.
- Chemical Instability: Changes in the postmortem environment, such as shifts in pH, can lead to the chemical degradation of analytes. For example, heroin is rapidly hydrolyzed to 6-monoacetylmorphine (6-MAM) and then to morphine. Certain benzodiazepines and antidepressants are also prone to degradation.
- Evaporation: Volatile substances, like solvents or inhalants, can be lost through evaporation as tissues break down and body cavities are exposed to the environment.
Interpreting toxicology results from decomposed specimens is complex and fraught with uncertainty. The primary challenges stem from postmortem redistribution and the inability to establish a clear timeline.
- Postmortem Redistribution (PMR): This is a phenomenon where drugs move from areas of high concentration (like the liver, lungs, and heart) into the bloodstream after death. This process can artificially inflate blood drug concentrations, making it difficult to determine if a level was therapeutic or fatal. The effect varies greatly depending on the drug's chemical properties. For instance, drugs like antidepressants and opioids are highly susceptible to PMR.
- Site-Dependent Concentrations: Because of PMR, drug concentrations can vary significantly depending on where the sample was collected from the body. Blood from the heart may have a much higher concentration than blood from a peripheral site like the femoral vein.
- Formation of Metabolites: Decomposition can sometimes mimic the body's metabolic processes. For example, some microbes can convert one drug into another (e.g., amitriptyline to nortriptyline), complicating the interpretation of whether the deceased ingested the parent drug or the metabolite.
Laboratory Preservation Techniques
To mitigate these challenges, forensic laboratories employ specific collection and preservation techniques to maintain the integrity of toxicological specimens as much as possible.
- Refrigeration and Freezing: The most critical step is to slow down decomposition by keeping the body and collected specimens cool. Refrigeration at approximately 4°C (39∘F) slows enzymatic and microbial activity. For long-term storage, samples are frozen, typically at -20°C (−4∘F) or lower.
- Use of Preservatives: Chemical preservatives are often added to biological samples, especially blood and urine. Sodium fluoride is the most common preservative. It serves two purposes:
- Anticoagulant: It inhibits enzymes, like those involved in glycolysis.
- Antimicrobial: It prevents bacteria and yeast from growing, thus stopping the postmortem formation of ethanol and the degradation of drugs.
- Alternative Specimens: In highly decomposed cases where traditional samples like blood and urine are unavailable or unreliable, toxicologists may turn to alternative specimens. These can include hair, nails, bone, or vitreous humor (the fluid in the eye), as they are more resistant to decomposition.
Guideline for Specimen Collection and Preservation for Forensic Toxicology
