Relatively little is known about adverse events (AEs) - physical injury to a patient due to health care that requires some intervention - and the characterization of patient harm in the ED. Even less is known about the nature of AEs occurring in the ED vs. those occurring prior to arrival, ie, how often ED visits result from AEs occurring elsewhere in the health system. This is a secondary evaluation of data from a retrospective observational study testing the ED Trigger Tool at an urban, academic medical center (data from 10/1/2014 - 10/31/2015; 92,860 eligible visits). Patients aged ≥18 completing a visit were eligible for inclusion. We performed dual independent level 1 nurse review of 4,772 visits sampled from records with ≥1 of 97 candidate triggers (findings that increase the likelihood of an AE), followed by a confirmatory level 2 physician review of records with putative AEs. AE severity was characterized with the MERP index. AEs were categorized using the ED Taxonomy of Adverse Events and Near Misses, which includes categories, subcategories and modifier terms. Reviewers indicated whether AEs occurred in the ED or were present on arrival (POA). We present descriptive data, bivariate associations between patient sociodemographics, triggers and POA AEs detected and multivariable modeling identifying POA AEs. Analyses were conducted using SAS 9.4 and R. We identified 965 AEs, of which 614 were POA and 351 occurred in the ED. Patients with POA AEs were more often white (51% vs 40%) and older (median age 62 vs 55 ) compared to patients with AEs in the ED, with no difference in distribution by sex (53% female overall). Medication and Patient Care categories account for over 70% of both ED and POA AEs. POA AEs included more health care Associated Infections and Surgical/Procedural AEs, and fewer Device-related AEs than those occurring in the ED. Key differences are visible at the subcategory level. Medication-related hypotension is a common AE in the ED (23%) but is rarely POA (3%). Medication-related bleeding is a common POA AE (21%) but is rarely an ED AE (3%). Medication-related Allergic Reaction is common to both (∼10-13%). Eight EDTT triggers are associated with POA AEs. The joint presence of any two of these triggers occurs in 9.4% of all visits in the population, has a positive predictive value of 39.6% for POA AEs, and identifies POA AEs similar to those described in Table 1. The ED trigger tool is a promising new approach to quality and safety review and identifies a broad scope of event types. The majority (64%) of AEs detected in the ED are present on arrival. This underscores the role of the ED not only as a safety net for patients but also for patient harm that occurs elsewhere in the health system. This also demonstrates that it may be important for surveillance methods to distinguish between POA and ED events for internal quality improvement.Tabled 1ED (N=351 AEs)POA (N=614 AEs)CategorySubcategory% of AEsCategorySubcategory% of AEsMedication63.20%Medication58%Hypotension23.40%Bleeding21.20%Allergic Reaction10.30%Allergic Reaction13.80%Hypoxia9.40%Fall5.90%Other4.30%Other4.20%Nausea/Vomiting4.30%Hypotension3.40%Glycemic Event3.70%Delirium2.60%Bleeding3.40%Dizziness2.40%Delirium2.00%Patient Care14.30%Device13.40%Failure Monitor7.70%Infiltration/Extravasation13.10%Fall5.90%Other2.80%Patient Care17.10%Delayed Treatment2.00%Failure Monitor6.60%Delayed Treat.5.40%HCAI11.10%Other4.80%Pneumonia4.70%Fall3.10%Surgical Site Infection*2.80%Delayed Dx2.30%Glycemic Event2.00%Surgical/ Procedural11.10%Hypoxia2.00%Other3.70%Wound Complication**2.80%Surgical/ Procedural5.10%Ileus/Obstruction2.00%Bleeding2.00%Device5.00%HCAI<1%Infection2.00%Frequencies of AE categories/subcategories for POA and ED events. Rare subcategories are omitted for clarity; and an event may have more than one subcategory.HCAI - Healthcare Associated Infection*< 30 Days Post-Op; Up To 1 Year If Implanted Hardware Present**Non-infectious Open table in a new tab