Paracetamol - serum

5 mL blood in plain tube collected, ideally 4 hours after ingestion of the paracetamol.

A second specimen collected at least 2 hours later can be used to measure paracetamol half-life.

Toxic range: >1,300 µmol/L (195 mg/L) 4 hours after ingestion of paracetamol.

Paracetamol half-life:
<4 hours: low probability of liver damage
>12 hours: hepatic coma likely.

See Figure 2. Risk of liver damage.

Toxicity is more likely and occurs at lower plasma levels in patients with pre-existing liver disease.

Calculation of half-life (from two specimens) is also a useful guide to hepatotoxicity as prolongation of half-life is a good predictor of the risk of development of acute hepatocellular damage.