International Classification of Diseases (ICD) codes are used by many quality measures to identify cohorts of patients with disease positive states. Because emergency department (ED) patients are evaluated and managed in real-time according to their chief complaint, we hypothesize that quality measures utilizing a chief complaint-based denominator may be a more accurately reflect care delivered in the ED. In this study, we sought to compare patient demographics, disposition, and outcomes using 2 unique yet overlapping denominators using medical record data from a large health care network. Using electronic medical record data between 1/1/2017 - 4/13/2018 from 16 different emergency departments in Western Pennsylvania, we included all patients ≥ 18 years old, afebrile (temp ≤ 38°C), for whom an ECG and troponin were ordered in the ED. We excluded any patient not evaluated in an in-network outpatient office within 365 days before the index ED visit in order to allow reliable estimation of patient comorbidities in our patient cohort. We then identified ED patients with chest pain using 2 different methods. In our chief complaint cohort, patients were selected using a text string matching algorithm. In our ICD cohort, patients were selected using a predefined set of ICD-10 diagnostic codes for chest pain which matched the denominator of a quality measure currently used by the Emergency Quality Network (E-QUAL). Patient outcomes, including 30-day rates of acute myocardial infarction and mortality were determined from subsequent medical records, including office visits, and the national death index. We compared patient demographics, ED disposition, and outcomes between our 2 patient cohorts using a Pearson’s Chi-square test with alpha error rate of 0.05. Of 78,619 patients meeting our initial inclusion and exclusion criteria, 22,092 patients were identified using the chief-complaint algorithm while 21,800 patients were identified using the ICD coding method. Although there was no difference in overall admission rates between the chief complaint cohort and the ICD cohort, more patients in the ICD cohort were admitted to observation status (46.7 versus 41.4, p<0.001). In terms of 30-day outcomes, there was a higher rate of acute myocardial infarction and mortality in the chief complaint cohort relative to the ICD cohort (AMI: 2.0 versus 1.1, p<0.001; mortality: 0.8 versus 0.7, p=0.027). Similarly, there was a higher rate of diabetes mellitus and underlying coronary artery disease in the chief complaint cohort relative to the ICD cohort (diabetes: 20.6 versus 19.6, p=0.008; coronary artery disease: 18.7 versus 16.9, p<0.001). In comparison to a chief-complaint-based denominator, an ICD-based denominator may select for a lower risk ED population with a lower incidence of short-term morbidity and mortality.