Key investigations

Measure the serum paracetamol and alanine aminotransferase (ALT) concentrations in patients suspected of ingesting a toxic dose of immediate-release paracetamol. Plot the serum paracetamol concentration versus time since ingestion on the nomogram (Paracetamol toxicity treatment nomogram)—acute liver injury can occur when the serum paracetamol concentration is on or above the treatment nomogram line.

If less than 8 hours have elapsed since ingestion, measure the serum paracetamol and ALT concentrations within 4 to 8 hours of ingestion. The serum ALT concentration is not used to assess the need for acetylcysteine therapy, but is measured in this group as a baseline. Serum ALT concentration rises slowly after an acute ingestion of paracetamol, and may remain within the normal reference range for up to 24 hours.

Note: The serum ALT concentration is not needed to guide treatment with acetylcysteine when less than 8 hours have elapsed since ingestion of immediate-release paracetamol.

If more than 8 hours have elapsed since ingestion, measure the serum paracetamol and ALT concentrations on arrival. Both the serum paracetamol and ALT concentrations are required to guide treatment with acetylcysteine.

Note: Both the serum paracetamol and ALT concentrations are required to guide treatment with acetylcysteine when more than 8 hours have elapsed since ingestion of immediate-release paracetamol.

Other investigations that may be performed if the serum ALT concentration is abnormal include:

  • blood glucose concentration
  • serum electrolyte, urea and creatinine concentrations
  • INR
  • blood gas analysis.