Negative predictive value of acetaminophen concentrations within four hours of ingestion.

The objective was to ascertain whether acetaminophen (APAP) concentrations less than 100 μg/mL obtained between 1 and 4 hours after acute ingestion accurately predict a nontoxic 4-hour concentration.The authors performed a multicenter, prospective cohort study involving five emergency departments (EDs) participating in the ToxIC Research Network. Data were collected from May 2009 to December 2011. Patients with APAP concentrations <100 μg/mL drawn between 1 and 4 hours after acute ingestions, and concentrations drawn 4 or more hours after ingestions, were included in the study. Exclusion criteria included initial concentration >100 μg/mL, initial APAP concentration drawn prior to 1 hour, two undetectable APAP concentrations, the second concentration drawn prior to 4 hours, and unknown time of ingestion. Toxic concentrations 4 or more hours after ingestion were defined as concentrations that plotted above the 150 μg/mL line on the Rumack-Matthew nomogram.Data were collected on 83 patients who met inclusion criteria. Of the 83 patients with APAP concentrations <100 μg/mL between 1 and 4 hours, one patient (1.2%) had a ≥ 4-hour toxic concentration. Negative predictive value (NPV) for an APAP concentration <100 μg/mL obtained between 1 and 4 hours after an acute ingestion was 98.8% (95% confidence interval [CI] = 93.5% to 99.8%).An APAP concentration of <100 μg/mL obtained between 1 and 4 hours after ingestion has a high NPV for excluding toxic ingestion. We do not recommend reliance on concentrations obtained between 1 and 4 hours to exclude toxicity, because of a potential false-negative rate of 6.5%.