Paracetamol overdose: timing the level
The case
A 29-year-old woman presents to the emergency department 3 hours after deliberately taking an estimated 30 g of paracetamol. She is drowsy but rousable. Heart rate is 88/min, blood pressure 118/72 mmHg, respiratory rate 16/min and oxygen saturation 98% on air. The abdomen is soft and non-tender. ALT is 22 U/L (0–40), INR 1.0 (0.9–1.1) and creatinine 70 micromol/L (60–110). A plasma paracetamol concentration taken on arrival, 3 hours after ingestion, is 60 mg/L. The registrar asks for your recommendation. What is the most appropriate next step?
Options
- ARepeat the paracetamol level at 4 hours post-ingestion
- BGive activated charcoal and start oral acetylcysteine now
- CStart intravenous acetylcysteine immediately
- DUrgent liver transplant unit assessment
- EDischarge home with safety advice
Think it through before you read on. Which single option is best, and why are the other four wrong?
Show the answer and explanation
Answer
A. Repeat the paracetamol level at 4 hours post-ingestion
Why this is the right answer
Paracetamol overdose is the most common deliberate poisoning in Australia, and the decision to treat depends on a timed level. The Australian and New Zealand paracetamol poisoning guideline (published in the Medical Journal of Australia, 2020 update) uses a nomogram whose treatment line starts at 4 hours after ingestion. Levels taken before 4 hours cannot be plotted, because absorption may not be complete and the true peak is not yet known.
This woman's level was taken at 3 hours. It cannot be interpreted. The correct action is to take another level at 4 hours (or as soon as possible after 4 hours) and plot it. A 4-hour level at or above the treatment line (150 mg/L on the Australian nomogram) means acetylcysteine is started; a level below the line, with a normal ALT, means treatment is not needed.
Because she is within 8 hours of ingestion, there is time to wait for the result. Acetylcysteine given within 8 hours prevents liver damage almost completely, so the delay costs nothing. If the 4-hour level will not be available by 8 hours post-ingestion, acetylcysteine is started empirically and stopped if the level is below the line.
Activated charcoal is useful within 2 hours of a large ingestion (or up to 4 hours for very large or modified-release ingestions), but at 3 hours after a standard formulation it is unlikely to help and is not the priority.
A very large ingestion (over 30 g or more than 500 mg/kg) may need a higher acetylcysteine dose; check the guideline. A mental health assessment is required once she is medically stable.
Why the other options are wrong
- B
Charcoal is past its useful window at 3 hours, and treatment should not be started on an uninterpretable level.
- C
Empirical acetylcysteine before a 4-hour level is only needed if the result will not be back within 8 hours of ingestion.
- D
Transplant referral is for established liver failure, not a patient with normal ALT and INR at 3 hours.
- E
A 3-hour level of 60 mg/L may cross the treatment line at 4 hours; discharge without a valid level is unsafe.
High-yield takeaway
Paracetamol levels before 4 hours cannot be plotted: repeat at 4 hours, and start acetylcysteine if the result will not be back within 8 hours of ingestion.
Reference: Guidelines for the Management of Paracetamol Poisoning in Australia and New Zealand (MJA) (2020)
Common questions
Why can't a 3-hour paracetamol level be used?
Absorption may not be complete and the nomogram treatment line only starts at 4 hours, so an earlier level cannot be plotted.
When should acetylcysteine be started without a level?
If the 4-hour result will not be available within 8 hours of ingestion, or the timing of ingestion is unknown or staggered.
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