Clinical features and factors associated with emergency department transfer among outpatients.

Unplanned emergency department (ED) transfer from outpatient clinics represents an important escalation point that may reflect early physiological deterioration and limited ambulatory capacity for monitoring and urgent interventions. We conducted a retrospective matched case-control study of consecutive adult outpatients treated at our institution between January 1, 2021 and December 31, 2024. Cases were outpatients requiring ED transfer during or shortly after the index outpatient encounter, confirmed by transfer documentation and same-day ED registration. Controls were outpatients discharged without ED transfer and were selected using 1:1 matching by outpatient department/specialty, calendar time of visit, sex, and age within 5 years. The final analytic cohort included 2184 cases and 2184 matched controls (4368 patients); 106 encounters were excluded based on prespecified criteria. Abnormal vital signs (35.1%) and cardiopulmonary symptoms (30.3%) were the most common primary indications for ED transfer. Compared with controls, cases exhibited higher temperature, heart rate, respiratory rate, and blood pressure and lower SpO2. In multivariable logistic regression, variables independently associated with ED transfer included shorter symptom duration, cardiopulmonary or infection-related presenting complaints, diabetes mellitus, chronic pulmonary disease, and acute physiological derangements, particularly higher temperature and respiratory rate, lower peripheral oxygen saturation (SpO2), elevated systolic blood pressure, and altered mental status. These findings indicate that ED transfer from outpatient settings was associated primarily with objective markers of acute instability and selected comorbidity profiles. Causal inference or direct use as a prospective decision rule should not be drawn from this retrospective case-control analysis.