FM13.{7,10} | Analytical Toxicology & Bedside Tests — Summary & Reflection

KEY TAKEAWAYS

Bedside tests provide rapid, qualitative guidance at the point of care. Key tests: ferric chloride urine (purple = salicylates — false positive with phenothiazines); ECG (QRS widening = TCA/Na-channel block; QTc >500 ms = K-channel block/torsades risk); pupil size (miosis = OP/opioid; mydriasis = anticholinergic/sympathomimetic); urine immunoassay strips (class screen — confirm by GC-MS before court use); pulse oximetry is unreliable in CO poisoning — co-oximetry required; Wood's lamp urine (ethylene glycol — fluorescein-containing antifreeze only).

FSL analytical workflow: sample receipt with chain-of-custody verification → preliminary examination → extraction → screening (TLC, immunoassay — presumptive only) → confirmation + quantification (GC-MS for volatile/organic compounds; LC-MS/MS for non-volatile, thermally labile, high-MW compounds; AAS for metallic poisons).

TLC: silica plate, Rf value, visualisation reagents (Dragendorff for alkaloids); screening only — Rf values not unique. GC: heated column, carrier gas, FID or MS detector; gold standard for volatile organics and drugs; GC-MS provides unique mass spectrum fingerprint. LC/LC-MS: liquid mobile phase, HPLC column; for non-volatile, polar, thermally labile compounds (opioid metabolites, paracetamol, paraquat); LC-MS/MS = highest specificity. AAS: hollow cathode lamp (element-specific), flame or graphite furnace; quantifies metals only (arsenic, lead, mercury, thallium); cannot identify organic compounds; hair segmental AAS gives arsenic exposure timeline.

Medico-legal interpretation: chain of custody must be intact; post-mortem redistribution elevates post-mortem blood levels of lipophilic drugs (femoral > cardiac blood; vitreous humour unaffected); TLC positive must be confirmed by independent method before citation in court.

REFLECT

The TCA-overdose patient from the opening scenario survived because the ECG was interpreted correctly within 2 minutes. Reflect: which of the bedside tests discussed in this module could you perform right now with the equipment in a typical Indian district hospital emergency department, and which require specialist equipment? If the FSL for your region has a 6-week backlog for GC-MS confirmation, what is the practical consequence for a homicidal poisoning investigation? And at a training level: should medical students be taught to perform and interpret an emergency ECG in poisoning as a core clinical skill? Make the case with reference to the competencies from this module.