Acetaminophen toxicity is characterized by a phased clinical presentation:

Stage I (0-24 hours)

• Patients are typically asymptomatic or may experience non-specific symptoms such as nausea, vomiting, and malaise.
• Liver function tests (LFTs) are generally within normal limits.

Stage II (24-72 hours)

• This phase is marked by the onset of hepatotoxicity.
• Clinical signs may include right upper quadrant abdominal pain.
• Laboratory abnormalities emerge, including elevated serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), and prolonged prothrombin time (PT) and international normalized ratio (INR).
• In severe cases, evidence of nephrotoxicity (elevated BUN, creatinine, oliguria) or pancreatitis (elevated serum amylase and lipase) may be present.

Stage III (72-96 hours)

• This represents the peak of hepatotoxicity and is characterized by overt signs of liver failure, including jaundice, coagulopathy, and encephalopathy.
• Severe cases may progress to acute renal failure, metabolic acidosis, and multi-organ failure.
• The highest risk of mortality occurs during this stage.

Stage IV (4 days to 2 weeks)

• In survivors, this is the recovery phase.
• Hepatic regeneration begins and typically completes within a few weeks.

The differential diagnosis for APAP-induced hepatotoxicity is broad and includes toxicologic and non-toxicologic etiologies.

• Toxicologic: Carbon tetrachloride, hepatotoxic mushrooms (e.g., Amanita phalloides), halothane, idiosyncratic drug reactions, pennyroyal oil, and iron poisoning.
• Non-Toxicologic: Viral hepatitis (A, B, C, Epstein-Barr virus, cytomegalovirus), inborn errors of metabolism, hepatobiliary disease, and Reye syndrome.

Routine Laboratory Studies:

• Serum glucose, electrolytes, AST, ALT, PT, bilirubin, albumin, blood urea nitrogen (BUN), and creatinine.
• Arterial Blood Gas (ABG)
• Electrocardiogram (ECG)
• Salicylate level
• Pregnancy test for all women of childbearing age.
• A baseline AST can be used to screen for pre-existing hepatic disease.
• Serial monitoring of ALT and INR every 12 hours is recommended for any patient with initial ALT elevation.
• Significant elevation of ALT, AST, bilirubin, and alkaline phosphatase, along with prolonged PT/INR, indicates the degree of liver injury.

Serum Acetaminophen Level:

• A serum APAP concentration should be obtained at 4 hours post-ingestion. This value is applied to the Rumack-Matthew nomogram to guide treatment decisions.

Nomogram Application:

• The nomogram is used for acute, single ingestions of immediate-release preparations.
• Its use is contraindicated in cases of chronic ingestion, sustained-release formulations, or in patients with risk factors for hepatotoxicity (e.g., chronic alcohol use).
• In these cases, the risk of hepatotoxicity is higher, and the treatment line would be significantly lower.

Admission Criteria:

• Patients requiring N-acetylcysteine (NAC) therapy should generally be admitted.
• Select patients with early presentation and no evidence of liver injury may be managed in an emergency department observation unit.
• Patients with acute liver failure require admission to an intensive care unit (ICU) and may necessitate transfer to a facility with liver transplantation capabilities.

Stabilization:

• Airway, breathing, and circulation (ABC) must be evaluated and stabilized, particularly in patients presenting in Stage II or III.

Decontamination:

• Activated charcoal (1 g/kg, max 50 g) is recommended within 1 hour of ingestion.
• Its use may be extended up to 4 hours post-ingestion, especially for sustained-release formulations.

Antidote: N-Acetylcysteine (NAC):

• Early administration of NAC is paramount for preventing hepatotoxicity.

Indications for NAC Therapy:

• Serum APAP concentration on or above the "treatment" line of the nomogram at 4 hours or more post-ingestion.
• Suspected single ingestion of >150 mg/kg or a total dose of >7.5 g in an adult, when serum APAP levels will not be available within 8 hours.
• Unknown time of ingestion with a serum APAP concentration >10 µg/mL (66 µmol/L).
• Any history of APAP ingestion with evidence of liver injury (e.g., elevated transaminases).
• Delayed presentation (>24 hours) with a history of excessive APAP ingestion and laboratory evidence of liver injury.

Dosage and Administration:

IV NAC:

• The preferred route for patients with vomiting, pregnancy, contraindications to oral administration (e.g., ileus, obstruction), or evidence of hepatic failure.

20-Hour Protocol:

• Loading dose: 150 mg/kg infused over 1 hour.
• Second infusion: 50 mg/kg over the next 4 hours.
• Third infusion: 100 mg/kg over the final 16 hours.

48-Hour Protocol:

• A more prolonged regimen (140 mg/kg loading, followed by 12 doses of 70 mg/kg over 4 hours each) may be superior when treatment is initiated 16-24 hours post-ingestion.

Adverse Reactions:

• Anaphylactoid reactions (bronchospasm, hypotension, urticaria) can occur and are managed with epinephrine and corticosteroids.

Oral NAC:

• Loading dose of 140 mg/kg, followed by 17 doses of 70 mg/kg every 4 hours.

NAC in Pregnancy:

• NAC crosses the placenta and should be administered to a pregnant woman with the same indications as non-pregnant patients.
• A newborn to a mother with APAP toxicity should receive a 48-hour course of IV NAC.

Indications:

• ECTR effectively removes APAP and is recommended in specific scenarios:
• APAP concentration >1000 µg/mL (>6620 µmol/L) without NAC administration.
• Altered mental status, metabolic acidosis, and an elevated lactate with APAP >700 µg/mL (>4630 µmol/L) and no NAC.
• Altered mental status, metabolic acidosis, and an elevated lactate with APAP >900 µg/mL (>5960 µmol/L) even if NAC has been initiated.

Management:

• Intermittent hemodialysis is the preferred modality.
• NAC therapy should be continued at an increased rate during ECTR.

• Antiemetics (e.g., metoclopramide, ondansetron).
• Monitoring and treatment of hypoglycemia.
• Vitamin K may be administered to correct coagulopathy.

King's College Hospital Criteria for Transplantation:

• Arterial pH <7.3, or arterial lactate >3.0 mmol/L after adequate fluid resuscitation.
• OR if all three of the following occur within a 24-hour period:
• Serum creatinine >300 µmol/L.
• PT >100 seconds (INR >6.5).
• Grade III/IV hepatic encephalopathy.

Definition

• Supratherapeutic dosing for an extended period.
• Children <6 years: >200 mg/kg over 8-24 hours; >150 mg/kg/day for >2 days; or >100 mg/kg/day for >3 days.
• Patients >6 years: >10 g or >200 mg/kg (whichever is less) over 24 hours; or >6 g or >150 mg/kg (whichever is less) per day for >2 days.

Treatment

• Indications for NAC in chronic toxicity include:
• Elevated AST or ALT (monitor for 36 hours after the last dose).
• APAP level >10 µg/mL.
• Symptomatic patients with normal transaminases.

• Note: Serum APAP concentration does not reliably correlate with toxicity in chronic ingestions. Diagnosis is primarily based on history and evidence of liver injury.

Discharge Criteria

From the Emergency Department

• Certainty regarding the time of ingestion.
• Serum APAP level below the toxic threshold.
• Completion of a psychiatric evaluation.

From the Hospital

• After a full course of NAC.
• Normal or improving liver and renal function tests.
• Psychiatric evaluation completed if indicated.

Patient Education

• Patients should be educated on the potential dangers of over-the-counter medications.
• Warn patients against the simultaneous use of multiple APAP-containing products.
• Highlight the risk of toxicity from inappropriate formulation substitution.

• Evaluation: Failure to determine the accurate time of ingestion, or to account for the impact of sustained-release formulations or anticholinergic medications on the accuracy of the 4-hour APAP concentration.
• Treatment: Failure to decontaminate patients within the appropriate timeframe. Prematurely discontinuing NAC therapy in patients with elevated transaminases or detectable APAP concentrations.