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NAC Reaction Management — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyNAC Reaction ManagementFRCA Final

A 33-year-old woman weighing 65 kg presents to the emergency department 4 hours after deliberately ingesting 15 g of paracetamol. Her ALT is normal and her plasma paracetamol concentration is above the treatment line. Fifteen minutes after starting the loading infusion of intravenous acetylcysteine, she develops pruritic urticaria, flushing and nausea. She is speaking normally, with no dyspnoea, wheeze or angioedema. Her blood pressure is 115/70 mmHg and SpO2 is 98% on air. What is the most appropriate action?

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Correct answer: A — Temporarily stop acetylcysteine, give intravenous chlorphenamine, and restart at a lower infusion rate once the reaction has resolved

This is a mild acetylcysteine-associated anaphylactoid infusion reaction occurring during the loading dose, without airway, breathing or circulatory compromise. The infusion should be stopped temporarily and the urticaria and flushing treated with an H1 antihistamine such as intravenous chlorphenamine. Once symptoms have resolved, acetylcysteine should be restarted at a lower rate because antidotal treatment remains indicated. It should not be abandoned permanently. Intramuscular adrenaline is appropriate if significant or progressive airway, breathing or circulatory features develop, but is not required for this isolated cutaneous and gastrointestinal reaction. Continuing at the same rate risks progression. Activated charcoal does not replace acetylcysteine, and hydrocortisone alone neither provides immediate control nor justifies withholding the antidote.

Reference: Electronic Medicines Compendium, Acetylcysteine 200 mg/ml Injection, SmPC sections 4.4 and 4.8, updated 10 September 2025: https://www.medicines.org.uk/emc/product/3447/smpc