Purpose: In 2012, a revised guideline for acute acetaminophen overdose was proposed in the UK, recommending that the treatment threshold should be lowered to 100 mcg/ml at 4 hours after ingestion without risk stratification of hepatotoxicity. However, the poison centers in some developing countries do not have laboratory resources to provide serum drug levels in time. The primary aim of the study is to evaluate the cut-off value of reported dose per kilogram to determine when N-acetylcysteine treatment is warranted under the revised guideline. Methods: Data were collected retrospectively from the toxicology registry of an urban emergency medical center between 1st January 2010 and 30th June 2017. Inclusion criteria were single acute overdose of more than 75 mg/kg in 15 hours from ingestion and over 14 years of age. Subgroups were created by 25 mg/kg increments of reported dose, then sensitivity, specificity, positive predictive value and negative predictive value were calculated for the cutoff values of 100 mg/kg, 125 mg/kg, 150 mg/kg and 175 mg/kg for toxic serum level over ‘100-treatment line’. Results: A total of 99 patients were enrolled in the study; 24 patients showed toxic serum levels (24.2%). Zero of 17 patients with an ingestion dose under 100 mg/kg showed toxic level (0%), and 0 of 15 under 125 mg/kg (0%), 2 of 14 under 150 mg/kg (14.3%), and 4 of 12 under 175 mg/kg (33.3%) had toxic levels. The higher the ingested dose per kilogram of weight, the higher the frequency of the toxic serum concentration on the first test (χ2 test for trend, χ2=22.66, p-value<0.001) and the sensitivity of each value was 100%, 100%, 92% and 76%. Conclusion: In acute single acetaminophen intoxication, the ingestion dose of 100 mg/kg of weight will be useful in determining the need for the N-acetylcysteine antidote in the indigent laboratory environment.