Residency · Residency · Emergency Medicine

Acetaminophen and Salicylate Toxicity

Acetaminophen (APAP) Toxicity

Epidemiology

Acetaminophen is the most common cause of acute liver failure in both the United States and the United Kingdom, and it is the most common drug involved in intentional self-poisoning. The toxic dose is generally considered to be greater than 150 mg/kg, or more than 7.5 to 10 grams in a single adult ingestion. Approximately 50 percent of all acute liver failure cases in the US are attributable to acetaminophen.

Metabolism and Pathophysiology

Under normal conditions, about 90 percent of acetaminophen is metabolized through glucuronidation and sulfation into non-toxic metabolites, with 5 percent excreted renally. The remaining 5 percent is metabolized by the cytochrome P450 enzyme CYP2E1 into NAPQI, a highly reactive and toxic intermediate. NAPQI is normally conjugated by glutathione and rendered harmless. In overdose, the glucuronidation and sulfation pathways become saturated, shunting more drug through CYP2E1 and producing excessive NAPQI. Once glutathione stores are depleted, free NAPQI binds to hepatocyte proteins and causes centrilobular hepatic necrosis. N-acetylcysteine (NAC) works by replenishing glutathione and serving as a direct glutathione substitute.

Risk Factors for Enhanced Toxicity

Certain patients are at higher risk of toxicity at lower doses. Chronic alcohol use induces CYP2E1 while simultaneously depleting glutathione. Malnutrition and fasting deplete glutathione stores. Medications that induce CYP2E1, such as isoniazid, phenobarbital, and carbamazepine, increase the production of NAPQI.

Clinical Stages (Acute Single Ingestion)

Acetaminophen toxicity from an acute single ingestion progresses through four stages. Stage I (0 to 24 hours) is deceptively benign: patients may have nausea, vomiting, anorexia, and malaise, or they may be completely asymptomatic with normal laboratory values. This is the dangerous "well-appearing" window. Stage II (24 to 72 hours) brings right upper quadrant pain, rising AST and ALT (AST typically rises first), increasing INR, and rising creatinine. Nausea may actually improve during this phase, creating a deceptive sense of clinical improvement. Stage III (72 to 96 hours) represents peak hepatotoxicity, with fulminant hepatic failure manifesting as coagulopathy, jaundice, encephalopathy, lactic acidosis, renal failure, and multiorgan failure. AST and ALT may exceed 10,000 IU/L. Stage IV (4 to 14 days) is the recovery phase in survivors. Complete hepatic recovery is typical if the patient survives the acute insult.

Rumack-Matthew Nomogram

The Rumack-Matthew nomogram applies only to acute single ingestions with a known time of ingestion. A serum APAP level is drawn at four hours post-ingestion and plotted on the nomogram. The treatment line begins at 150 mcg/mL at four hours, and some institutions use the lower 100 mcg/mL line for an extra safety margin. If the level falls above the treatment line, NAC should be started. The nomogram does not apply to chronic or repeated supratherapeutic ingestions, unknown time of ingestion, extended-release formulations, or staggered overdoses.

When to Treat Without the Nomogram

NAC should be started empirically when the time of ingestion is unknown and the patient has a detectable APAP level with elevated transaminases, when chronic ingestion has produced any transaminase elevation, when a massive ingestion raises any concern, and whenever there is doubt. NAC is extremely safe, and the risk of not treating far exceeds the risk of the drug itself.

N-Acetylcysteine (NAC)

The traditional IV protocol is a 21-hour, three-bag regimen. Bag 1 delivers 150 mg/kg in 200 mL D5W over 60 minutes. Bag 2 delivers 50 mg/kg in 500 mL D5W over 4 hours. Bag 3 delivers 100 mg/kg in 1000 mL D5W over 16 hours, for a total of 300 mg/kg over 21 hours. Anaphylactoid reactions (which are not true allergies) including flushing, urticaria, and bronchospasm are most common with the first bag and are managed by slowing or pausing the infusion, treating with antihistamines, and then resuming.

A modified two-bag protocol is gaining acceptance due to a lower rate of anaphylactoid reactions and simpler administration. It delivers 200 mg/kg in 500 mL D5W over 4 hours (bag 1) followed by 100 mg/kg in 1000 mL D5W over 16 hours (bag 2), totaling 300 mg/kg over 20 hours. By eliminating the rapid first bag, which is responsible for most adverse reactions, this protocol has been adopted by several major toxicology centers.

The oral protocol is a 72-hour regimen with a loading dose of 140 mg/kg PO followed by 70 mg/kg PO every four hours for 17 additional doses, totaling 1330 mg/kg. Vomiting is common and should be managed with ondansetron. The advantage is the absence of anaphylactoid reactions, but the longer duration is a disadvantage.

Massive APAP Ingestion

When the APAP level exceeds 500 mcg/mL at four hours or the reported ingestion exceeds 30 to 40 grams, higher NAC doses may be needed, and some toxicologists recommend doubling the third bag. The APAP level should be repeated four hours after the first; if it is still rising, absorption is ongoing. For extended-release preparations, a second level should be obtained four to eight hours after the first. Metabolic acidosis with a massive ingestion is an ominous early sign that reflects mitochondrial dysfunction developing before hepatotoxicity becomes apparent.

NAC Endpoints and Disposition

At the end of the NAC infusion, AST, ALT, INR, creatinine, and APAP level should be checked. NAC can be safely stopped when the APAP level is undetectable, transaminases are trending down or normal, INR is normal, and the patient is clinically well. If any of these criteria remain abnormal, NAC should be continued by repeating the third bag or its equivalent until criteria are met. Referral for liver transplant evaluation should follow the King's College Criteria: a pH below 7.3 after resuscitation, or the combination of grade III-IV encephalopathy, INR above 6.5, and creatinine above 3.4 mg/dL.

Salicylate Toxicity

Pharmacology

Aspirin (acetylsalicylic acid) is converted to its active metabolite, salicylic acid. The therapeutic level is 10 to 20 mg/dL, toxicity begins above 30 mg/dL, severe toxicity above 60 mg/dL, and levels above 80 to 100 mg/dL are potentially lethal. At toxic doses, salicylate follows zero-order kinetics, meaning small additional amounts cause disproportionately large increases in serum levels.

Pathophysiology

Salicylate stimulates the medullary respiratory center, producing an early respiratory alkalosis. It also uncouples oxidative phosphorylation, generating a metabolic acidosis with elevated lactate and an elevated anion gap. The classic acid-base pattern is therefore a mixed respiratory alkalosis and anion gap metabolic acidosis. Salicylate impairs glucose metabolism, and CNS hypoglycemia can occur even when serum glucose is normal. Increased capillary permeability leads to non-cardiogenic pulmonary edema resembling ARDS. Platelet function is also inhibited.

Acute vs. Chronic Toxicity

Acute salicylate toxicity is usually intentional, easier to diagnose, and correlates well with serum levels. Chronic toxicity is far more dangerous at the same serum level, often misdiagnosed, and typically occurs in elderly patients taking supratherapeutic doses over time. It presents with altered mental status, tinnitus, and metabolic derangements, and carries higher mortality because of delayed recognition.

Clinical Presentation

Early symptoms include nausea, vomiting, tinnitus, tachypnea, and diaphoresis. Moderate toxicity produces agitation, confusion, fever, and the mixed respiratory alkalosis with metabolic acidosis. Severe toxicity manifests as seizures, coma, pulmonary edema, cerebral edema, cardiovascular collapse, and renal failure. Tachypnea out of proportion to the clinical appearance is a key early clue to the diagnosis.

Management

Initial stabilization includes IV fluid resuscitation, as salicylate-toxic patients are often volume depleted. Dextrose should be administered even with a normal serum glucose, because the CNS may be hypoglycemic despite normal peripheral levels. Activated charcoal is effective, especially within one to two hours, and multi-dose activated charcoal should be considered for large ingestions. Intubation should be avoided if at all possible.

Urinary alkalinization is accomplished with sodium bicarbonate, 150 mEq in 1 liter of D5W infused at 150 to 200 mL per hour, targeting a urine pH of 7.5 to 8.0. This ionizes salicylate in the urine, trapping it for renal excretion. Potassium must be monitored closely because hypokalemia prevents effective urine alkalinization through H+/K+ exchange in the kidney. Serum pH should be kept between 7.45 and 7.55 and should not exceed 7.55.

Intubation in severe salicylate poisoning is one of the most dangerous procedures in emergency medicine. These patients compensate for their metabolic acidosis through massive hyperventilation with minute ventilation of 20 to 30 liters per minute. Even brief apnea during rapid sequence intubation causes a rapid drop in pH, which shifts salicylate into the CNS and can precipitate cardiovascular collapse and death. If intubation is unavoidable, the patient should be pre-oxygenated, a bicarbonate bolus of 1 to 2 mEq/kg should be given before induction, the ventilator should be set to match the patient's pre-intubation respiratory rate and tidal volume (requiring very high minute ventilation), and emergent hemodialysis should be arranged. Some experts recommend BiPAP as a bridge to avoid intubation entirely.

Hemodialysis is indicated per EXTRIP guidelines when the salicylate level exceeds 90 mg/dL in acute ingestion or 60 mg/dL in chronic ingestion, when altered mental status is present, when a new oxygen requirement or pulmonary edema develops, when renal failure impairs elimination, when there is failure to improve despite alkalinization, when pH is below 7.2 despite resuscitation, or when clinical deterioration occurs despite appropriate treatment.

Monitoring

Serial salicylate levels should be obtained every two hours until trending down, with awareness that bezoar formation can occur with large ingestions. Blood gas analysis will reveal the characteristic mixed acid-base disturbance. The basic metabolic panel should be monitored for anion gap, potassium, bicarbonate, glucose, and renal function. A CBC and coagulation studies should also be obtained.

<image>The Rumack-Matthew nomogram for acetaminophen toxicity. The Y-axis shows serum APAP concentration (mcg/mL) on a logarithmic scale from 1 to 1000. The X-axis shows hours post-ingestion from 0 to 36. The treatment line begins at 150 mcg/mL at 4 hours and slopes downward to approximately 4.7 mcg/mL at 24 hours. A second, lower "possible toxicity" line starting at 100 mcg/mL at 4 hours is shown as a dotted line (used in some countries for added safety margin). The zone above the treatment line is labeled "treat with NAC." Annotations note that the nomogram only applies to acute single ingestions with a known time of ingestion, and list scenarios where it does NOT apply: chronic ingestion, unknown timing, extended-release formulations, staggered overdoses.</image>

<image>A diagram showing the pathophysiology of acetaminophen metabolism in normal and overdose states. The left panel shows normal metabolism: APAP undergoes glucuronidation (45%) and sulfation (45%) to non-toxic metabolites; 5% is metabolized by CYP2E1 to NAPQI, which is conjugated by glutathione to non-toxic mercapturic acid. The right panel shows overdose: glucuronidation and sulfation pathways are saturated (shown as blocked arrows); increased CYP2E1 metabolism produces excess NAPQI; glutathione stores are depleted (shown as empty tank); free NAPQI binds to hepatocyte proteins causing centrilobular necrosis. An arrow shows where NAC intervenes by replenishing glutathione and directly binding NAPQI.</image>

<image>A clinical management flowchart for salicylate toxicity. Starting with suspected salicylate ingestion leading to: obtain salicylate level, ABG/VBG (look for mixed respiratory alkalosis and metabolic acidosis), BMP (anion gap, potassium), and glucose. Mild toxicity (level 30-50 mg/dL): IV fluids, activated charcoal, urinary alkalinization with sodium bicarbonate, serial levels. Moderate toxicity (level 50-90 mg/dL): aggressive alkalinization, correct hypokalemia, dextrose for CNS hypoglycemia, consider nephrology consultation. Severe toxicity (level greater than 90 mg/dL or any end-organ dysfunction): emergent hemodialysis, bicarbonate bolus, AVOID intubation if possible (red warning box explaining the danger of apnea and pH drop during RSI). A separate warning box emphasizes: chronic salicylism is more dangerous than acute at the same level — have a lower threshold for aggressive treatment.</image>

Clinical Pearls

Acetaminophen and salicylate levels should be checked in all intentional ingestions, because co-ingestion is common and both are treatable if caught early. A patient who looks well in Stage I of APAP toxicity may develop fatal hepatic failure in Stage III, so treatment should be guided by the nomogram rather than clinical appearance. NAC is extremely safe, and when in doubt, it should be started because the risk of not treating far exceeds the risk of the drug. The two-bag NAC protocol is simpler and produces fewer anaphylactoid reactions than the traditional three-bag protocol. Chronic salicylate toxicity is more dangerous than acute toxicity at the same serum level and is frequently misdiagnosed as sepsis, altered mental status, or encephalopathy in the elderly. Intubation in severe salicylate poisoning is one of the most dangerous procedures in emergency medicine because loss of compensatory hyperventilation causes rapid acidemic cardiovascular collapse. Hypokalemia must be corrected before urinary alkalinization will be effective. Dextrose should be given to salicylate-toxic patients even with normal serum glucose, because the CNS can be hypoglycemic despite normal peripheral levels.

References

• Rumack BH, Matthew H. Acetaminophen poisoning and toxicity. Pediatrics. 1975;55:871-876.
• Prescott LF, et al. Intravenous N-acetylcysteine: the treatment of choice for paracetamol poisoning. BMJ. 1979;2:1097-1100.
• Wong A, et al. A modified two-bag intravenous acetylcysteine regimen for paracetamol overdose. Clin Toxicol. 2016;54:670-674.
• Palmer BF, Clegg DJ. Salicylate toxicity. NEJM. 2020;382:2544-2555.
• Juurlink DN, et al. Extracorporeal treatment for salicylate poisoning (EXTRIP). Ann Emerg Med. 2015;66:165-181.

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