Interpretation of Clinical and Postmortem Chemistry and Toxicology
Toxicology interpretation depends on specimen, timing, tolerance, and postmortem change. Core concepts: therapeutic, toxic, and lethal ranges; the difference between blood and serum concentrations; postmortem redistribution (central blood overestimates drugs that diffuse from liver/lungs); site-specific sampling (femoral blood preferred, vitreous for alcohol and electrolytes, urine for metabolites, bile for drugs with biliary excretion); and the role of metabolites (e.g., 6-acetylmorphine for heroin, cocaethylene for combined cocaine and alcohol).
Traps: treating a postmortem level as antemortem; ignoring tolerance in chronic opioid or benzodiazepine users; attributing death to a drug found at a therapeutic concentration when another lethal process exists; misreading alcohol levels from contaminated or fermented specimens; and failing to distinguish carbon monoxide (carboxyhemoglobin, cherry-red lividity, fire scene) from cyanide (bitter almond odor, industrial fire). Correlate vitreous electrolytes and glucose with clinical dehydration or diabetic ketoacidosis.
Worked example
A decedent has femoral blood morphine 0.05 mg/L and a history of chronic opioid therapy. Rule of thumb: interpret in context; a low level in a tolerant user may not be lethal, whereas the same level in an opioid-naive person may be significant. Check 6-acetylmorphine to confirm heroin use and review the scene for paraphernalia.
Always ask: which specimen, what site, what timing, and what tolerance?