title: Negative predictive value of acetaminophen concentrations within four hours of ingestion.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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%).
  
  
  CONCLUSIONS
  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%.
authors.affiliation: Department of Pediatrics and Emergency Medicine, Indiana University School of Medicine, Indianapolis, IN.
authors.forename: Blake A
authors.initials: BA
authors.lastname: Froberg
authors.forename: Kerry J
authors.initials: KJ
authors.lastname: King
authors.forename: Theomal D
authors.initials: TD
authors.lastname: Kurera
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Monte
authors.forename: Jane M
authors.initials: JM
authors.lastname: Prosser
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Walsh
authors.forename: Robert H
authors.initials: RH
authors.lastname: Riffenburgh
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Rusyniak
authors.forename: David A
authors.initials: DA
authors.lastname: Tanen
mesh_terms: D000082:Acetaminophen / Q000097:blood* / Q000506:poisoning; D000293:Adolescent; D000328:Adult; D000368:Aged; D018712:Analgesics, Non-Narcotic / Q000097:blood* / Q000506:poisoning; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D062787:Drug Overdose; D004636:Emergency Service, Hospital; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011446:Prospective Studies; D055815:Young Adult
chemical_list: D018712:Analgesics, Non-Narcotic; D000082:Acetaminophen