The Clinical Use of the Plasma Acetaminophen Halflife in NAC-treated Acetaminophen Overdose

ABSTRACT

PURPOSE: A plasma acetaminophen halflife of more than 4 hours has been correlated with hepatotoxicity in acetaminophen overdosing not treated with an antidote. However, the acetaminophen halflife has not been studied in patients receiving the antidote N-acetylcysteine (NAC). METHODS: Ninety-eight (98) patients with acetaminophen overdoses, all of whom were treated with N-acetylcysteine were studied. A minimum of 2 plasma acetaminophen valu e s > 2.0 microgram/ml were available for calculating the acetaminophen halflife, assuming first-order kinetics. RESULTS: Overall, the median acetaminophen halflife was 6.2 hours (range, 1.15~103.9 hours). Sixty-two (62) patients with no hepatotoxicity (AST 1,000 U/L) without acute hepatic failure, and 6 patients with hepatotoxicity (AST > 1,000 U/L) with hepatic failure had acetaminophen halflives of 3.7 hours (range, 1.15~23.2 hours), 5.9 hours (range, 1.96~26.2 hours), 6.3 hours (range, 2.19~15.38 hours), and 32.8 hours (range, 5.48~103.9 hours), respectively (p<0.05). A receiver operating characteristic curve analysis showed that an acetaminophen halflife of 5.19 hours provided better discrimination (sensitivity=69.6%, specificity=84.0%, accuracy=80.6%). CONCLUSION: The acetaminophen halflife correlates well with the degree of liver damage in patients treated with N-acetylcysteine. Longer halflives reflect a greater toxic effect on the liver.