Emergency department-initiated outpatient parenteral antimicrobial therapy in Taiwan: A retrospective cohort study on clinical outcomes and cost analysis

Background: ED-initiated outpatient parenteral antimicrobial therapy (OPAT) aims to reduce admissions and relieve ED boarding. In practice, however, OPAT may be used as a “middle-ground” alternative to oral therapy, complicating assessment of its true value. Because studies using matched inpatient and outpatient comparators with longitudinal endpoints are scarce, we evaluated the effectiveness, safety, and medical costs of an ED-initiated OPAT program in Taiwan. Methods: This retrospective cohort study analyzed ED-initiated OPAT patients from two teaching hospitals (2017–2019). Using coarsened exact matching, we created two comparison groups: inpatient-matched (assessing effectiveness) and outpatient-matched (assessing safety). The primary outcome was net hospital days saved over 30 days, derived from daily hospital-prevalence trajectories. Secondary outcomes were 14-day cumulative incidence of ED revisits/readmission and a stratified cost analysis over the treatment course. Results: Of 1409 OPAT patients, 986 were matched. In the inpatient-matched cohort (n = 416), OPAT saved a net 8.9 hospital-days per patient over 30 days. In the outpatient-matched cohort (n = 570), OPAT showed a transiently higher risk of return visits at day 7 (risk difference +5 %; p = 0.008) without increases in severe adverse events or 14-day readmissions. OPAT reduced costs by NT$34,367 per patient when substituting for hospitalization but increased costs when compared with standard outpatient care. Conclusions: For appropriately selected patients requiring admission-level care, ED-initiated OPAT can be a cost-saving substitute for hospitalization. Given limited safety data, benefits remain conditional on rigorous patient selection to avoid overuse and on structured early reassessment to mitigate early revisit risks.