Question

A 24-year-old woman presents to the emergency department after taking repeated doses of paracetamol for severe dental pain over the previous 36 hours. She is unable to remember the exact number of tablets taken or the timing of each dose. She currently feels well and has no abdominal tenderness. Blood tests are sent, including liver function tests, INR, creatinine and a plasma paracetamol concentration.

What is the most appropriate next step in management?

A. Wait for the paracetamol concentration and plot it on the treatment nomogram
B. Discharge if the plasma paracetamol concentration is below the treatment line
C. Start intravenous N-acetylcysteine
D. Give activated charcoal regardless of the timing of ingestion
E. Repeat the paracetamol concentration after 24 hours

Answer

C. Start intravenous N-acetylcysteine

Explanation

This is a staggered paracetamol overdose with uncertain timing. The key examination point is that the paracetamol treatment nomogram cannot be used reliably when paracetamol has been taken repeatedly over a prolonged period or when the time of ingestion is uncertain.

In a single, acute overdose with a clearly known ingestion time, the plasma paracetamol concentration should be measured at least 4 hours after ingestion and plotted against the treatment nomogram. A concentration obtained before 4 hours cannot reliably exclude significant toxicity.

However, in a staggered overdose, there is no single meaningful “time zero” from which to plot the concentration. A relatively low paracetamol level may therefore be falsely reassuring even though substantial hepatic injury may be developing.

For this reason, intravenous N-acetylcysteine (NAC) should be started when significant staggered overdose or uncertain timing is suspected rather than waiting to see whether the concentration crosses the nomogram treatment line.

Paracetamol poisoning can be particularly deceptive because patients may initially be completely asymptomatic. Early symptoms, if present, are usually nonspecific, such as nausea, vomiting and abdominal discomfort.

Hepatotoxicity typically becomes evident later. Patients may develop right upper-quadrant tenderness, markedly elevated ALT/AST, prolonged INR, jaundice, hypoglycaemia, metabolic acidosis, acute kidney injury and, in severe cases, acute liver failure with encephalopathy.

Important investigations in a late or potentially significant presentation include:

• Paracetamol concentration
• ALT/AST
• INR or prothrombin time
• Creatinine and electrolytes
• Blood glucose
• Lactate and acid-base status in seriously unwell patients

N-acetylcysteine replenishes hepatic glutathione, allowing detoxification of the toxic paracetamol metabolite NAPQI. It is most effective when given early, but it still provides benefit in late presentations and established hepatic injury.

Therefore, a patient presenting late with evidence of paracetamol-induced liver injury, particularly a raised ALT, should receive NAC even if the circulating paracetamol concentration is low or undetectable.

Activated charcoal may be considered soon after a substantial acute ingestion, particularly within the first few hours, but it does not replace NAC when NAC is indicated.

Summary for quick exam revision

Paracetamol overdose may initially be completely asymptomatic, so absence of symptoms does not exclude serious toxicity. Nausea and vomiting may be the only early manifestations. In a single acute ingestion with a known time, measure the plasma paracetamol concentration at least 4 hours after ingestion and plot it on the treatment nomogram. A level taken before 4 hours cannot reliably determine the need for treatment. If the concentration is above the treatment line, intravenous N-acetylcysteine should be given. The nomogram is only valid for a single acute ingestion with a reliably known time. It must not be used for staggered overdoses or when the time of ingestion is uncertain. In staggered or uncertain-time overdose, a low paracetamol concentration may be falsely reassuring, so NAC should be started when clinically indicated rather than relying on the nomogram. NAC works mainly by replenishing glutathione and enhancing detoxification of the toxic metabolite NAPQI. Patients presenting late may develop right upper-quadrant pain, very high transaminases, coagulopathy, jaundice, hypoglycaemia, acute kidney injury and encephalopathy. Important investigations in late presentations include ALT/AST, INR, creatinine, glucose and, in severe cases, lactate and acid-base status. Raised ALT in a late presentation strongly suggests hepatic injury and is an indication to start or continue NAC irrespective of a low paracetamol concentration. NAC is most effective when given early but remains beneficial even after hepatic injury has developed. The key exam distinction is therefore: known single ingestion → 4-hour level and nomogram; staggered or uncertain ingestion → nomogram unreliable and treat with NAC when significant toxicity is suspected.