Objective: To identify the predicting factors of unpredicted adverse event and in-hospital mortality after admission to the emergency department observation unit (EDOU).

Materials and Methods: A retrospective study of all non-traumatic patients who were admitted to an emergency department based observation unit between July 1, 2015 and June 30, 2016. The main outcome measurements were unpredicted adverse event and the secondary outcome was in-hospital mortality.

Results: Of the 1,037 patients enrolled, 52 (5%) developed unpredicted adverse event. Predictors of unpredictable adverse event included age older than 65 years (odds ratio [OR] 4.45, 95% confidence interval [CI] 2.15 to 9.20), the emergency department (ED) qSOFA score more than two (OR 3.15, 95% CI 1.48 to 6.70), cirrhotic patients (OR 3.83; 95% CI 1.45 to 10.10), cardiovascular patients (OR 6.28, 95% CI 3.18 to 12.41), initial body temperature at EDOU hotter than 38℃ (OR 2.40; 95% CI 1.12 to 5.11), and initial pulse rate greater than 90/minute (OR 2.63, 95% CI 1.34 to 5.15). The hospital mortality at 3-day and 28-day of adverse event group were 0.1% (p<0.001) and 1.3% (p<0.001), respectively.

Conclusion: In determining the appropriateness of patients selected for an EDOU: elderly patients, cirrhotic patients, cardiovascular patients, and an ED qSOFA score more than 2, should be the qualifying criteria. Patients placed into the EDOU were more likely to develop unpredictable adverse events if they had a fever and/or tachycardia in the EDOU.

Keywords: Adverse event, Observation unit, Emergency department