Which laboratory tests should be obtained in a case of paracetamol (acetaminophen) poisoning?

Laboratory Tests in Paracetamol Poisoning

Obtain a serum paracetamol (acetaminophen) concentration drawn between 4-24 hours post-ingestion and hepatic aminotransferase levels (ALT/AST) to risk-stratify patients and guide N-acetylcysteine (NAC) treatment decisions.

Essential Laboratory Tests

Primary Tests for Risk Stratification

Serum paracetamol concentration is the cornerstone test that must be obtained and plotted on the Rumack-Matthew nomogram to determine hepatotoxicity risk 1. The timing is critical:

• Draw between 4-24 hours post-ingestion for accurate nomogram interpretation
• Concentrations drawn before 4 hours cannot be reliably interpreted
• The nomogram stratifies patients into "no risk," "possible risk," or "probable risk" zones 1

Hepatic aminotransferases (ALT/AST) should be obtained to screen for existing hepatotoxicity 1. This is particularly important when:

• Time of ingestion is unknown or unreliable
• Patient presents >24 hours post-ingestion
• Detectable paracetamol level exists but ingestion history is unclear
• Suspected repeated supratherapeutic ingestions 1

Additional Laboratory Tests

INR (International Normalized Ratio) helps assess hepatic synthetic function and severity of hepatotoxicity 1. This becomes critical in patients with established liver injury or hepatic failure.

Renal function tests should be obtained as paracetamol toxicity can cause acute kidney injury in severe cases.

Clinical Algorithm for Test Ordering

Known Single Acute Ingestion (<24 hours)

1. Obtain serum paracetamol level at 4 hours post-ingestion (or immediately if >4 hours)
2. Plot on Rumack-Matthew nomogram
3. Obtain baseline ALT/AST and INR
4. If above treatment line: initiate NAC and monitor aminotransferases

Unknown Time or Unreliable History

1. Obtain immediate serum paracetamol level
2. Obtain aminotransferases (ALT/AST) and INR
3. If any paracetamol is detectable AND aminotransferases are elevated: strongly consider NAC treatment 1
4. Normal aminotransferases in the ED do not exclude developing toxicity over subsequent hours 1

Presentation >24 Hours Post-Ingestion

1. Serum paracetamol level (nomogram not applicable, but guides ongoing treatment)
2. Comprehensive hepatic panel: ALT, AST, INR, bilirubin
3. Renal function
4. Consider NAC if any hepatotoxicity present 1

Critical Pitfalls to Avoid

Do not rely on the nomogram for:

• Ingestions <4 hours (levels unreliable)
• Unknown time of ingestion
• Extended-release formulations (prolonged absorption) 1
• Repeated supratherapeutic ingestions 1

Do not assume safety from normal aminotransferases alone when paracetamol is detectable with unclear history—hepatotoxicity may develop over the next several hours 1.

Rare cases exist where patients have elevated aminotransferases despite "no risk" classification on the nomogram, suggesting inaccurate history or increased individual susceptibility 1. Always consider alternative causes of hepatic injury in these cases.

Point-of-Care Testing Considerations

Rapid point-of-care tests for paracetamol exist with high sensitivity (98.5%) and can provide results approximately 2 hours faster than laboratory analysis 2, 3. These can expedite clinical decision-making, particularly for ruling out paracetamol overdose in patients with suspected mixed ingestions 2. However, laboratory confirmation remains the gold standard for treatment decisions.

Treatment Context

The laboratory results directly determine NAC administration, which reduces severe hepatotoxicity from 58% to <10% when given within 8-10 hours, and reduces mortality even when initiated up to 24 hours post-ingestion 1. Time is critical—every hour of delay increases hepatotoxicity risk, making rapid laboratory turnaround essential for optimal outcomes.