What are the initial general pathology tests for suspected unknown poisoning?

Initial General Pathology Tests for Suspected Unknown Poisoning

For suspected unknown poisoning, immediately obtain basic metabolic panel (electrolytes, creatinine, bicarbonate), calculate the anion gap, perform arterial or venous blood gas analysis, obtain serum osmolality with calculated osmol gap, measure serum glucose, and obtain an ECG. 1, 2

Core Laboratory Assessment

Acid-Base and Electrolyte Evaluation

• Obtain serum electrolytes, bicarbonate, and calculate the anion gap (Na+ + K+ - Cl- - HCO3-), as an elevated anion gap can indicate toxic alcohol ingestion (ethylene glycol, methanol), salicylates, or other metabolic poisons 3
• Perform arterial or venous blood gas analysis to assess pH, metabolic acidosis, and respiratory status, which helps identify toxidromes and guide management 3
• An anion gap >27 mmol/L strongly suggests severe toxic alcohol poisoning requiring urgent intervention 3

Osmolality Assessment

• Measure serum osmolality and calculate the osmol gap (measured osmolality minus calculated osmolarity), as an elevated osmol gap (>12 mmol/L) suggests toxic alcohol ingestion such as ethylene glycol or methanol 3
• The osmol gap is particularly useful early in toxic alcohol poisoning before metabolism to toxic metabolites has occurred 3

Metabolic Parameters

• Obtain serum glucose immediately in all patients with altered mental status, as hypoglycemia is rapidly reversible and can mimic poisoning 4
• Measure serum creatinine to assess renal function, which may be impaired by nephrotoxic agents or secondary to severe poisoning 2

Specialized Testing Based on Clinical Context

Carbon Monoxide Assessment

• Obtain carboxyhemoglobin (COHb) level by co-oximetry on arterial or venous blood in patients with suspected carbon monoxide exposure, altered mental status, or exposure history 3
• Standard pulse oximetry is unreliable for detecting CO poisoning, as COHb reads similarly to oxyhemoglobin at 660 nm, often showing falsely normal oxygen saturation despite significant CO poisoning 3
• Noninvasive pulse CO-oximetry has poor sensitivity (48%) and should not be relied upon; laboratory-based spectrophotometry is required for diagnosis 3

Electrocardiography

• Obtain a 12-lead ECG in all patients with suspected poisoning, particularly with chest pain, dysrhythmias, or ingestion of cardiotoxic agents (tricyclic antidepressants, beta blockers, calcium channel blockers, antidysrhythmics) 3, 2
• ECG findings may reveal QRS widening, QT prolongation, or dysrhythmias that guide antidote therapy 3

Acetaminophen and Salicylate Levels

• Obtain acetaminophen and salicylate levels in all patients with intentional overdose or unknown ingestion, as these are common, potentially lethal, and have specific antidotal therapy 5, 2, 4
• Acetaminophen levels should be drawn at least 4 hours post-ingestion to allow interpretation using the Rumack-Matthew nomogram 5

Additional Considerations

Toxic Alcohol Metabolites

• If available, obtain serum glycolate concentration in suspected ethylene glycol poisoning, as levels >12 mmol/L indicate severe poisoning requiring extracorporeal treatment 3
• Glycolate correlates with acute kidney injury and mortality better than ethylene glycol concentration itself 3

Lactate Measurement Caution

• Be aware that elevated glycolate can falsely elevate lactate measurements on some analyzers due to cross-reactivity, potentially masking the true severity of toxic alcohol poisoning 3

Common Pitfalls

• Do not rely on pulse oximetry alone in suspected CO poisoning; it will appear falsely normal 3
• Do not delay treatment while awaiting laboratory confirmation if clinical suspicion is high; initiate supportive care and specific antidotes (e.g., 100% oxygen for CO, naloxone for opioids) immediately 3, 4
• The anion gap is only useful for predicting toxic alcohol exposure when pre-test probability is high; indiscriminate use in low-probability cases yields poor predictive value 3
• COHb levels may be normal or low if significant time has elapsed since exposure or if oxygen therapy was initiated, but this does not exclude CO poisoning 3