What is the window for N-acetylcysteine administration after acetaminophen ingestion?

• A. 0-2 hours after ingestion
• B. 4-24 hours after ingestion
• C. 24-48 hours after ingestion
• D. 72 hours after ingestion

Answer: (B)

The window being tested is the period when the acetaminophen level can be used reliably to decide on NAC therapy with the Rumack-Matthew nomogram. Between four and twenty-four hours after ingestion, a single acute acetaminophen concentration correlates with risk of hepatotoxicity. If the level sits above the treatment line on the nomogram, N-acetylcysteine should be started to replenish glutathione and detoxify the reactive metabolite NAPQI, preventing liver injury.

Before four hours, levels aren’t fully reliable because absorption and distribution are still changing, so the nomogram isn’t applicable yet. After twenty-four hours, the nomogram loses validity for guiding treatment, and management relies more on evidence of liver injury (elevated enzymes, bilirubin, INR) and clinical status. NAC can still be used if there are signs of ongoing damage, but the nomogram-guided window is four to twenty-four hours.