Which blood tests should be obtained when poisoning is suspected?

Blood Tests for Suspected Poisoning

When poisoning is suspected, immediately obtain basic metabolic panel (electrolytes, glucose, BUN, creatinine), complete blood count, liver function tests (AST, ALT, bilirubin), arterial or venous blood gas with co-oximetry (for carboxyhemoglobin and methemoglobin), serum osmolality, lactate, and acetaminophen/salicylate levels regardless of history 1, 2.

Essential Initial Laboratory Panel

The following tests should be ordered stat for any suspected poisoning case:

Core Chemistry Tests

• Electrolytes (sodium, potassium, chloride, bicarbonate) - Calculate anion gap (normal <12 mEq/L). Elevated anion gap suggests toxic alcohols, salicylates, metformin, or cyanide 2, 3.
• Glucose - Hypoglycemia can mimic intoxication; hyperglycemia may indicate salicylate toxicity 2.
• BUN and creatinine - Assess renal function and calculate osmolar gap 2.
• Serum osmolality (measured) - Calculate osmolar gap by subtracting calculated osmolality from measured. Gap >10 suggests toxic alcohols (ethylene glycol, methanol) 4, 2.

Hematology

• Complete blood count - Leukocytosis, anemia, or hematocrit changes may indicate specific poisonings 5.

Hepatic Function

• AST, ALT, total bilirubin, total protein - Hepatotoxicity from acetaminophen or other agents 5, 6.

Critical Toxicology Tests

• Co-oximetry (arterial or venous blood) - Directly measures carboxyhemoglobin (COHb) and methemoglobin. Standard pulse oximetry is unreliable and will give falsely normal readings in carbon monoxide poisoning 7, 8. COHb >3-4% in nonsmokers or >10% in smokers confirms carbon monoxide exposure 7.
• Lactate - Elevated lactate (>10 mmol/L) with pH <7.20 suggests cyanide co-poisoning, especially in fire victims 9, 7.
• Acetaminophen level - Must be obtained in all overdose patients regardless of history, as it is common, treatable, and patients may not report it 1, 6.
• Salicylate level - Another common, treatable poisoning that requires specific testing 1.

Specialized Tests Based on Clinical Presentation

If Specific Toxins Suspected

• Digoxin level - If cardiac glycoside poisoning suspected 9.
• Lithium level - In patients on lithium therapy 1.
• Iron level - If iron ingestion suspected 1.
• Cholinesterase (RBC and plasma) - For organophosphate/carbamate poisoning 1.
• Ethanol level - Most common "date-rape drug"; can be measured in blood 10, 11.

For Date-Rape Drug Suspicion

Critical timing issue: These drugs have extremely short detection windows 10:

• Urine toxicology for flunitrazepam - Detectable only 24-48 hours post-ingestion 10.
• GHB testing - Must be collected within 12 hours or less; cleared rapidly 10.
• Ketamine - Detectable in urine 24-72 hours 10.
• Benzodiazepines and antihistamines - Also used to facilitate assault 10.

Collect specimens immediately, even before completing history/physical if date-rape drugs suspected 10. Many reference labs do not routinely test for these substances, so contact the lab beforehand to ensure proper specimen collection and handling.

Important Caveats

What NOT to Rely On

• Pulse oximetry - Cannot distinguish carboxyhemoglobin from oxyhemoglobin; readings will be falsely reassuring in CO poisoning 12, 8.
• Noninvasive CO-oximetry - Insufficient sensitivity for ruling out CO poisoning in symptomatic patients; requires blood co-oximetry confirmation 8.
• Standard urine drug screens - Have significant limitations in sensitivity/specificity and rarely change acute management 11, 1. Most emergency physicians do not rely on these for emergent decisions 1.

Critical Timing Considerations

• Toxic alcohol poisoning: Osmolar gap may normalize as parent compound metabolizes to toxic acids, while anion gap increases 4, 2.
• Acetaminophen: Use nomogram only for acute single ingestions with known time; modified-release or repeated supratherapeutic ingestions require different interpretation 6.
• Carbon monoxide: COHb levels decrease rapidly with oxygen therapy (half-life 60-90 minutes on 100% O₂); early measurement is critical 12, 7.

When to Suspect Cyanide Co-Poisoning

If arterial pH <7.20 AND lactate ≥10 mmol/L persisting after 30 minutes of 100% oxygen therapy in a fire victim, treat empirically for cyanide without waiting for confirmatory testing 9, 12, 7.

Consultation

Contact your regional poison control center or medical toxicologist immediately for guidance on specialized testing and interpretation 9, 1. Many poisonings require antidotes or treatments that most clinicians use infrequently, and expert consultation facilitates rapid, effective therapy 9.