Specimens and Screening
Forensic toxicology analyzes biological specimens for drugs, poisons, and alcohol to determine their role in injury or death. The medical examiner collects multiple specimens at autopsy: blood from the femoral vein (least affected by postmortem redistribution), urine (concentrates drugs over hours, useful for detecting recent use), vitreous humor (isolated from postmortem changes, excellent for alcohol), liver tissue (the major site of drug metabolism, concentrates many substances), bile (accumulates drugs excreted by the liver), and gastric contents (may reveal undigested pills or toxic substances). Initial screening uses immunoassay panels that detect drug classes—opiates, benzodiazepines, amphetamines, cannabinoids, cocaine metabolites—with results in hours. Positive screens must always be confirmed by more specific methods because immunoassays can produce false positives from structurally similar compounds.