What lab test detects acetaminophen overdose?

Detecting Acetaminophen Overdose

Serum acetaminophen concentration is the definitive laboratory test to detect acetaminophen overdose and should be obtained at least 4 hours after ingestion. 1, 2

Primary Assessment

• Obtain serum acetaminophen concentration at least 4 hours post-ingestion

  • Concentrations obtained earlier than 4 hours may be misleading as they may not represent maximum acetaminophen levels 2
  • Results are plotted on the Rumack-Matthew nomogram to determine risk of hepatotoxicity 3, 1

• If time of ingestion is unknown:

  • Obtain acetaminophen concentration immediately
  • Begin N-acetylcysteine (NAC) treatment while awaiting results 2

Additional Laboratory Tests

After confirming acetaminophen overdose, the following tests should be ordered to monitor for complications:

• Liver function tests:

  • Aspartate aminotransferase (AST)
  • Alanine aminotransferase (ALT)
  • Bilirubin
  • International normalized ratio (INR)

• Renal function and metabolic tests:

  • Creatinine
  • Blood urea nitrogen (BUN)
  • Electrolytes
  • Blood glucose
  • Arterial blood gases (in severe cases) 1, 2

Timing Considerations

• The Rumack-Matthew nomogram is only valid for single acute ingestions with known time of ingestion within 24 hours 3, 1
• For patients presenting >24 hours after ingestion, the acetaminophen level may be undetectable despite significant hepatotoxicity 4
• Peak serum transaminase activities usually occur 48-96 hours after acute ingestion 4

Special Situations

• For repeated supratherapeutic ingestions (RSTI), the nomogram cannot be used
• In these cases, obtain acetaminophen level and liver function tests to guide management 3, 5
• For extended-release formulations, consider obtaining a second acetaminophen concentration 8-10 hours post-ingestion 2

Practical Considerations

• The reported history of quantity ingested is often inaccurate and not a reliable guide to therapy 2
• Some emergency departments may use rapid acetaminophen detection kits when formal laboratory testing is not immediately available, though these are less accurate than standard laboratory methods 6
• Always interpret acetaminophen levels in context with clinical presentation and timing of ingestion 3, 1

Common Pitfalls

• Waiting too long to obtain the acetaminophen level (optimal timing is 4-8 hours post-ingestion)
• Relying on the nomogram for repeated ingestions or when time of ingestion is unknown
• Failing to obtain follow-up liver function tests even when initial acetaminophen levels are below the treatment line
• Not considering acetaminophen toxicity in patients with unexplained liver injury, even when acetaminophen is not reported in the history 3, 4

Remember that early detection and treatment with NAC within 8 hours of ingestion provides the best outcomes for preventing hepatotoxicity in acetaminophen overdose 1.