Paracetamol Overdose — Viva

Q1: Recognition and mechanism (3 min)^[1]

Examiner: A 22-year-old woman is brought to casualty 4 hours after ingesting 20 g of paracetamol in a suicide attempt. She is asymptomatic, ALT and INR normal. The 4-hour paracetamol level is 180 mg/L. What is your diagnosis, what is the molecular mechanism of toxicity, and what is the immediate management?^[1]

Candidate: The diagnosis is single acute paracetamol overdose. The 4-hour level of 180 mg/L is above both the UK 100 mg/L treatment line and the US/ANZ Rumack-Matthew 150 mg/L line, so the immediate management is to start IV N-acetylcysteine (NAC) with the standard 21-hour three-bag regimen (150 mg/kg over 1 h, then 50 mg/kg over 4 h, then 100 mg/kg over 16 h; total 300 mg/kg over 21 h), alongside activated charcoal 50 g orally (it is within 1 hour in this case - actually within 4 hours, charcoal is most useful within 1 h but can be given within 2 h) and serial monitoring of ALT, INR, creatinine, lactate, pH. The molecular mechanism: at therapeutic doses paracetamol is metabolised mainly by glucuronidation (40 to 67 percent) and sulfation (20 to 46 percent) to harmless conjugates. Only 5 to 10 percent is oxidised by CYP2E1 to the toxic intermediate NAPQI (N-acetyl-p-benzoquinone imine), immediately detoxified by glutathione (GSH) to a mercapturic acid. In overdose the safe pathways saturate, CYP2E1 metabolism surges, GSH is depleted (toxicity when GSH under 30 percent), and free NAPQI binds covalently to cysteine residues on hepatocyte proteins, damaging mitochondria, opening the mitochondrial permeability transition pore, and producing centrilobular (zone 3) necrosis - the histological signature. She is in Phase 1 (the 'well window'): asymptomatic with normal labs but NAPQI is already forming adducts - reassuring and discharging her would be the classic lethal pitfall.^[1]

Q2: The nomogram and its limits (3 min)^[1]

Examiner: When can you use the nomogram, and when is it INVALID?^[1]

Candidate: The nomogram plots serum paracetamol (y-axis, log scale) against hours post single acute ingestion (x-axis, linear). It can be used from 4 to 24 hours post a SINGLE ACUTE ingestion, provided the time of ingestion is known precisely. Above the treatment line = treat with NAC; below = no NAC (if well). The level MUST be drawn at 4 hours or later - levels before 4 h are uninterpretable because absorption is incomplete. The treatment line is 150 mg/L at 4 h (US Rumack-Matthew) or 100 mg/L at 4 h (UK/ANZ/Europe, post-2012 single line). The line is a straight extrapolation assuming a paracetamol half-life of roughly 4 h. The nomogram is INVALID in six situations: (1) staggered ingestion (doses over more than 1 to 2 h); (2) unknown time of ingestion; (3) modified-release paracetamol; (4) repeated supratherapeutic ingestion (RSI); (5) presentation more than 24 h post-ingestion; and (6) established hepatotoxicity. In ALL of these situations, treat empirically with the full NAC regimen - do not wait for a level or a nomogram decision.^[1]

Q3: NAC - mechanism, regimen, and adverse reactions (3 min)^[1]

Examiner: Tell me how NAC works, the dose, and the commonest reaction.^[1]

Candidate: Mechanism: NAC is a sulfhydryl (thiol) donor and glutathione precursor - it is hydrolysed to cysteine, the rate-limiting substrate for glutathione synthesis (via gamma-glutamylcysteine synthetase), replenishing GSH so that any further NAPQI is detoxified. NAC also acts as a direct GSH substitute, scavenges free radicals, and enhances hepatic microvascular blood flow. Keays 1991 showed it improves survival even when given AFTER hepatotoxicity has developed, so it is continued (or started) in established ALF. Regimen: IV 21-h three-bag - 150 mg/kg over 1 h (loading), then 50 mg/kg over 4 h, then 100 mg/kg over 16 h (total 300 mg/kg over 21 h). In children use smaller diluent volumes to avoid fluid overload. Adverse reaction: an anaphylactoid (non-IgE) reaction in 8.2 percent of 6455 treatment courses (75.4 percent cutaneous; 95.4 percent within the first 5 hours, around the loading infusion) - flushing, urticaria, pruritus, occasionally bronchospasm or hypotension. More common with LOW paracetamol levels, rapid loading, and atopy. Management: pause the infusion, give chlorphenamine 10 mg IV (or diphenhydramine), then resume at a slower rate. Do NOT abandon NAC - its life-saving benefit outweighs the reaction. Adrenaline is reserved for true anaphylaxis with hypotension/bronchospasm/hypoxia. NAC works by REPLENISHING GLUTATHIONE - it does NOT bind paracetamol directly. It is nearly 100 percent protective within 8 hours of a single acute ingestion.^[1]

Q4: Late presentation and King's College Criteria (3 min)^[1]

Examiner:^[3] A patient arrives 36 hours after a massive paracetamol overdose. ALT 7800 U/L, INR 5.8, creatinine 280 micromol/L, arterial pH 7.25 after 2 L crystalloid, lactate 5.0 mmol/L. He is confused and drowsy. What do you do?

Candidate: This is paracetamol-induced acute liver failure in Phase 3. He has been started on IV NAC (continue it - Keays showed benefit even in established ALF). I now apply the King's College Criteria (O'Grady 1989) for paracetamol-induced ALF - either arm met = urgent transplant referral at over 80 percent predicted mortality. Arterial pH under 7.3 after adequate fluid resuscitation is met (pH 7.25 after 2 L) - so this is an urgent transplant referral, regardless of the second arm. The second arm would require ALL THREE of INR over 6.5 (he has 5.8 - just under) AND creatinine over 300 (he has 280 - just under) AND grade III/IV encephalopathy. The pH arm alone is met, so refer. Critically, do NOT correct the INR with FFP or vitamin K before transplant assessment - this confounds the prognostic INR, which is the key KCC variable; correct coagulopathy only if actively bleeding or before an invasive procedure. Admission arterial lactate (his is 5.0) is one of the validated prognostic variables alongside age, conscious level, pH, creatinine, INR and cardiovascular failure. In parallel, manage the ALF: head elevation 30 degrees, mannitol 0.5 g/kg IV for cerebral oedema (after excluding renal failure and checking osmolality under 320), hypertonic saline to keep sodium 145 to 155, intubate for grade III/IV encephalopathy, treat sepsis with surveillance cultures and antibiotics if signs of infection.^[1]

Q5: Special populations - pregnancy and the chronic alcoholic (2 min)^[1]

Examiner: How does your management change in pregnancy, and in the chronic alcoholic?^[1]

Candidate: Pregnancy: paracetamol crosses the placenta; the fetal liver has lower CYP2E1 but lower glutathione reserves, so the fetus is at risk. NAC is safe in ALL trimesters, crosses the placenta, has NO teratogenicity, and should be started without delay at full dose (standard 150/50/100 mg/kg regimen). The threshold to treat is LOWER in pregnancy. Obstetric and fetal-medicine input. Paracetamol overdose is NOT an indication for termination; the maternal outcome is the principal determinant of fetal outcome. Chronic alcoholic: chronic ethanol INDUCES CYP2E1 (more NAPQI production) while fasting/malnutrition depletes glutathione - the combination makes the chronic alcoholic toxic at LOWER doses, even at therapeutic dosing (RSI pattern). I would consider using the LOWER (100 mg/L) treatment line even for single ingestions in such patients, but I do NOT use a separate 'alcohol nomogram'. NAC dosing is unchanged. I also assess for alcohol withdrawal and give thiamine 100 mg IV BEFORE any glucose (to prevent Wernicke encephalopathy). Enzyme inducers (rifampicin, phenytoin, carbamazepine, isoniazid) similarly lower the threshold.^[1]

Q6: Prognosis and prevention (2 min)^[1]

Examiner: Tell me about prognosis, and the single most effective population-level prevention.^[1]

Candidate: The prognosis is excellent with early NAC: hepatotoxicity falls from roughly 50 to 60 percent untreated to under 5 percent when NAC is given within 8 hours, and mortality is near zero. Prognosis worsens with later presentation - NAC at 8 to 16 h gives roughly 15 to 30 percent hepatotoxicity, 16 to 24 h gives 30 to 50 percent - but NAC remains beneficial even after hepatotoxicity develops (Keays 1991: 48 percent versus 20 percent survival in established ALF). Established paracetamol ALF meeting King's College Criteria has over 80 percent mortality without transplant; emergency transplant reduces this to under 30 percent one-year. Predictors of poor outcome: delayed presentation, staggered ingestion, enzyme inducers, alcohol misuse, malnutrition, presentation already in ALF, pH under 7.3, INR over 6.5, creatinine over 300, grade III/IV encephalopathy, lactate over 3.5. The liver RECOVERS completely in survivors - no cirrhosis - because zone 1 hepatocytes are preserved and regenerate. The single most effective population-level prevention is the 1998 UK pack-size legislation (maximum 16 tablets in non-pharmacy, 32 in pharmacy), which reduced paracetamol-related deaths and liver unit admissions by roughly 20 to 40 percent. No comparable federal limit exists in the US, where unintentional supratherapeutic ingestion contributes a larger share of ALF.^[1]

References3ShowHide

1. [1]Smilkstein MJ, Knapp GL, Kulig KW, Rumack BH. Efficacy of oral N-acetylcysteine in the treatment of acetaminophen overdose. N Engl J Med, 1988.PMID 3059186
2. [2]Yarema M, Chopra P, Sivilotti MLA, et al. Anaphylactoid Reactions to Intravenous N-Acetylcysteine during Treatment for Acetaminophen Poisoning. J Med Toxicol, 2018.PMID 29423816
3. [3]O'Grady JG, Alexander GJ, Hayllar KM, Williams R. Early indicators of prognosis in fulminant hepatic failure. Gastroenterology, 1989.PMID 2490426