- Consensus recommendations for acute acetaminophen exposure include plotting an acetaminophen concentration at ≥ 4 h post ingestion on the Rumack-Matthew nomogram to determine the need for acetylcysteine treatment. We studied the frequency of acetaminophen concentrations drawn within 4 h post ingestion and whether the Rumack-Matthew nomogram was properly used in making acetylcysteine treatment decisions.This was a retrospective, observational case series at three regional poison centers of acute acetaminophen exposures between 1/1/13 and 12/31/13. Cases were analyzed for demographics, timing of acetaminophen concentrations, and application of the Rumack-Matthew nomogram in acetylcysteine initiation or termination.1,123 cases of acute acetaminophen exposure were reviewed. Of 520 acute acetaminophen exposure cases presenting < 4 h post ingestion, 323 (62%) had a pre-4-hour acetaminophen concentration measured and 197 (38%) did not. Those with a known pre-4-hour acetaminophen concentration were less likely to have a 4-hour acetaminophen concentration (59% vs. 93%) or an acetaminophen concentration within 8 h (87% vs. 99%) and were more likely to be treated with acetylcysteine (29% vs. 17%) and less likely to be treated based on the Rumack-Matthew nomogram (72% vs. 97%).Patients with a known exposure time and presenting within 4 h of acetaminophen ingestion had a pre-4-hour acetaminophen concentration obtained 62% of the time. Pre-4-hour acetaminophen concentrations cannot be used to determine the need for acetylcysteine therapy and are associated with an increased likelihood of not obtaining optimally timed acetaminophen concentrations and acetylcysteine management not based on the proper application of the Rumack-Matthew nomogram. Current practice results in additional cost, unnecessary treatment, potential adverse medication effects, and the possibility of non-treatment of patients at risk of hepatotoxicity.