N-Acetylcysteine Infusion Protocol for Paracetamol Poisoning
Administer N-acetylcysteine (NAC) intravenously using a total dose of 300 mg/kg divided over 21 hours as three separate infusions, starting immediately if the patient presents within 8 hours of ingestion with a toxic acetaminophen level, or immediately regardless of level if presenting after 8 hours. 1
Standard FDA-Approved Three-Bag Regimen
The FDA-labeled protocol consists of 1:
- Loading dose: 150 mg/kg in 200 mL diluent over 1 hour (60 minutes)
- Second dose: 50 mg/kg in 500 mL diluent over 4 hours
- Third dose: 100 mg/kg in 1000 mL diluent over 16 hours
Total treatment time: 21 hours Total dose: 300 mg/kg
Dilution Requirements
NAC is hyperosmolar (2600 mOsmol/L) and must be diluted before administration in 1:
- Sterile water for injection
- 0.45% sodium chloride (half-normal saline)
- 5% dextrose in water
When to Start NAC
For Acute Ingestion with Known Time
If presenting <8 hours post-ingestion 2, 1:
- Obtain acetaminophen level at 4+ hours post-ingestion
- Plot on Rumack-Matthew nomogram
- Start NAC if level is at or above the "possible toxicity" line (25% below the original treatment line)
- Do not start NAC if level is below the possible toxicity line and drawn >4 hours post-ingestion
If presenting >8 hours post-ingestion 1:
- Start NAC immediately without waiting for levels
- Obtain acetaminophen concentration to guide continuation
- Efficacy diminishes progressively after 8 hours, but NAC should never be withheld even if 15-24 hours post-ingestion
For Unknown Time or Delayed Presentation
Start NAC immediately if 1:
- Time of ingestion is unknown
- Acetaminophen level cannot be obtained within 8 hours
- Clinical evidence of acetaminophen toxicity exists
- Any uncertainty about hepatotoxicity risk
Continuing or Stopping NAC After 21 Hours
After completing the standard 21-hour protocol 1:
Check acetaminophen level, ALT/AST, and INR. Continue NAC if:
- Acetaminophen levels remain detectable
- ALT/AST are still rising
- INR remains elevated
For massive overdose, co-ingestions, or pre-existing liver disease, strongly consider extended treatment 1.
Recent evidence suggests early cessation at 12-13 hours may be safe in highly selected low-risk patients (ALT <40 IU/L at presentation and at 12 hours, acetaminophen <20 mg/L at 12 hours) 3, though this represents a deviation from standard FDA protocol and should only be considered with toxicology consultation.
Special Populations
Repeated Supratherapeutic Ingestion (RSI)
The Rumack-Matthew nomogram does not apply to RSI 1.
Start NAC immediately and obtain acetaminophen concentration plus liver function tests to guide treatment duration 1.
High-Risk Patients
Consider treating even with "non-toxic" levels in 2:
- Chronic alcoholism
- Malnutrition
- CYP2E1 enzyme inducers (e.g., isoniazid)
- Prolonged fasting
Extended-Release Formulations
Obtain a second acetaminophen level at 8-10 hours post-ingestion if the 4-hour level is below the treatment line 1.
Critical Timing Considerations
The 8-hour window is crucial 2:
- Patients treated within 8 hours: 0% severe hepatotoxicity
- Treated 8-10 hours: minimal hepatotoxicity
- Treated 10-16 hours: 29% severe hepatotoxicity
- Treated 16-24 hours: 62% severe hepatotoxicity
This dramatic increase in hepatotoxicity with delayed treatment underscores why NAC should be started immediately in any patient presenting >8 hours post-ingestion, even before obtaining levels.
Adverse Reactions and Management
Hypersensitivity reactions occur in approximately 14-15% of patients 1, 4, most commonly:
- Transient skin erythema or urticaria during loading dose
- Wheezing, shortness of breath
- Hypotension, bronchospasm
Management 1:
- Immediately discontinue infusion if serious reaction occurs
- Treat the reaction (antihistamines, bronchodilators, fluids)
- Carefully restart NAC at a slower rate after symptoms resolve
- Most reactions (91%) are mild and do not require discontinuation
Common Pitfalls
- Waiting for levels when presenting >8 hours: Start NAC immediately
- Using the nomogram for RSI or unknown time: These scenarios require immediate treatment
- Stopping at 21 hours with detectable acetaminophen or rising transaminases: Continue NAC
- Not diluting NAC: The hyperosmolar solution must be diluted to prevent complications
- Permanently stopping NAC after mild allergic reaction: Most can be restarted at slower rates
Alternative Regimens
While the FDA-approved 21-hour three-bag regimen remains standard in the United States 5, alternative protocols exist:
- 12-hour SNAP protocol (300 mg/kg over 12 hours): Shows similar efficacy with fewer adverse reactions 6, 7
- Two-bag regimens: Demonstrate fewer administration errors with comparable outcomes 8
However, the FDA-approved three-bag protocol should be used unless institutional protocols specify otherwise, as it has the most extensive safety data and regulatory approval 1.