Lower incidence of anaphylactoid reactions to N-acetylcysteine in patients with high acetaminophen concentrations after overdose.

ABSTRACT

BACKGROUND: Mechanisms responsible for anaphylactoid reactions to N-acetylcysteine (NAC) are poorly understood, and acetaminophen itself may play an important role. The present study examined the relationship between serum acetaminophen concentrations and risk of anaphylactoid reactions. METHODS: Prospective study of adverse reactions to NAC administered according to standardized clinical protocols in patients who present to hospital after acute acetaminophen overdose. Subgroups were defined by serum acetaminophen concentrations 0 to 100 mg/L, 101 to 150 mg/L, 151 to 200 mg/L, 201 to 300 mg/L, and >300 mg/L. RESULTS: There were 362 patients, and anaphylactoid reactions occurred in 14.9%. Anaphylactoid reactions occurred less commonly in patients with high serum acetaminophen concentrations (p = 0.046 by Cochran-Armitage trend test) and high equivalent 4 h acetaminophen concentrations (p = 0.004). DISCUSSION: High serum acetaminophen concentrations were associated with fewer anaphylactoid reactions, suggesting that these might in some way be protective. The biological basis needs further exploration so as to allow a better understanding of the mechanisms responsible for adverse reactions to NAC treatment.