Post-mortem drug immunoassays (PMDIs) are laboratory tests used in forensic toxicology to detect drugs or their breakdown products in biological samples collected after death. PMDIs serve as a screening tool, providing initial indications of drug exposure.
Understanding Post-Mortem Drug Immunoassays
PMDIs are a type of immunoassay, an analytical technique that uses the specific interaction between an antigen and an antibody to identify substances. The antigen is the drug or its metabolite, and the antibody is a specialized protein designed to bind to that substance. When a drug or metabolite is present, it binds to the antibodies, triggering a measurable signal.
These tests are considered screening tools. They primarily indicate the presence or absence of a drug class rather than providing precise quantities. PMDIs are designed for speed and efficiency, allowing for rapid processing of many samples and detection of a wide range of substances. While sensitive, they are not definitive for identification or quantification, meaning they do not tell you exactly how much of a substance was present or its exact chemical structure.
When PMDIs Are Used
PMDIs are employed in forensic investigations to help determine the circumstances surrounding a death. They are useful in cases where drug involvement is suspected, such as apparent overdoses or unexplained sudden deaths. Identifying the presence of drugs can provide insights into potential contributing factors to the cause of death.
PMDI results assist forensic pathologists and toxicologists in understanding the physiological effects a substance might have had. For instance, detecting a drug class could suggest intoxication or impairment that led to an accident. These tests are part of a broader toxicological analysis, guiding further, more specific investigations into the deceased’s toxicological profile.
The PMDI Process and Detected Substances
Post-mortem samples for PMDI analysis are collected during an autopsy. Common biological specimens include blood, urine, and vitreous humor (fluid from the eye). Tissue samples, such as liver or brain tissue, can also be used, especially when fluid samples are limited or degraded. These samples undergo preparation to extract or convert target substances for testing.
The prepared sample is introduced to a test system containing antibodies specific to various drug classes. PMDIs screen for a broad spectrum of substances, including:
Opioids (like morphine or fentanyl)
Benzodiazepines (such as diazepam or alprazolam)
Stimulants (like cocaine or amphetamines)
Cannabinoids (such as THC)
What PMDI Results Reveal
A positive PMDI result indicates that a substance belonging to a particular drug class was detected in the post-mortem sample. Conversely, a negative result suggests that the targeted substances were not present at detectable levels. It is important to remember that PMDIs are screening tests and do not provide specific identification or precise quantification of the substance. For example, a positive result for “opioids” does not specify whether it was morphine, codeine, or another opioid.
Due to the possibility of false positives or cross-reactivity with structurally similar compounds, confirmatory testing is always necessary after a positive PMDI result. Techniques like gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) are used for this purpose. These advanced methods can precisely identify the specific drug and measure its concentration in the sample. Interpretation of post-mortem drug levels can be complex due to factors like post-mortem redistribution, where drug concentrations can change in different body compartments after death. Therefore, determining impairment or intent from post-mortem levels requires careful consideration by forensic toxicologists in conjunction with other investigative findings.