Investigations (toxicology)

#completed #carded

Related FACEM curriculum (2022) learning objectives:

• ME 3.8.1.4(f) Principles of assessment of toxicological presentations in the ED, including: Investigations (ECG: sodium channel blockade, QT prolongation)

Screening Tests

• Detect occult toxic ingestions and allows initiation of early treatment.
• **Recommended Tests
  • 12-lead ECG:
    • Evaluates rate, rhythm, PR interval, QRS interval, QT interval, and dominant R wave in aVR.
    • Identifies cardiac conduction abnormalities (e.g., tricyclic antidepressant cardiotoxicity).
  • Bedside BGL:
    • Detects hypoglycaemia, especially in patients with altered mental status.
  • Serum Paracetamol Level:
    • Essential due to prevalence of APAP (paracetamol) poisoning.
    • Early detection prevents hepatic failure and death through N-acetylcysteine administration.
    • Screening level can be performed at presentation (not delayed to 4 hours).

Rationale for Paracetamol Screening

• Common in deliberate self-poisoning (up to 15% of cases in Australasia).
• Cost-effective due to potential life-saving interventions.
• A non-detectable paracetamol level >1 hour post-ingestion excludes significant poisoning.
• Timed Paracetamol Levels: Used when poisoning is suspected for additional risk assessment.

Screening for Other Agents

• Salicylate:
  • Not routinely screened due to its decline and easily recognizable symptoms (e.g., acid-base disturbances).
• Tricyclic Antidepressants:
  • Complications usually within 2–4 hours.
  • ECG preferred for reflecting clinical effects.

Minimal Investigations

• For young, healthy patients with normal mental status and vitals:
  • ECG, bedside BGL, and serum paracetamol are often sufficient.
  • Routine tests (e.g., electrolytes, LFTs, coagulation studies) generally unnecessary.

Specific Investigations

• Indications include:
  • Refining risk assessment or prognosis.
  • Confirming/excluding specific poisonings or differential diagnoses.
  • Identifying complications or indications for antidotes/enhanced elimination.
  • Monitoring therapy or defining endpoints for interventions.

Useful Drug Levels

• Assist in risk assessment or management for:
  • Carbamazepine
  • Digoxin
  • Ethanol, Ethylene glycol, Methanol
  • Iron
  • Lithium
  • Methotrexate
  • Paracetamol
  • Phenobarbitone
  • Salicylate
  • Theophylline
  • Valproic acid

Urine Drug Screens

• Rarely alter acute management due to:
  • False positives/negatives.
  • Positive results without clinical symptoms usually irrelevant.
  • Assays may not detect expected drugs (e.g., 'opiates' do not detect synthetic opioids such as fentanyl and methadone)