BACKGROUND: Unplanned re-attendance at the emergency department (ED) of patients with systemic inflammatory response syndrome (SIRS) remains unclear, and studies exploring health service factors related to unplanned re-attendance are limited. This study aimed to analyze the 30-day incidence of unplanned re-attendance in the ED following discharge and determine factors affecting unplanned re-attendance, including sociodemographic and clinical characteristics, agent, and the health service delivery system in patients with SIRS. METHODS: This study employed a cross-sectional and prospective cohort study design. The sample comprised 900 patients and 14 ED supervisors. This study was conducted between February 1, 2021 and July 30, 2022 at 14 hospitals in Thailand. Data were collected using a standardized questionnaire and information from medical records. RESULTS: The majority of the sample met two SIRS criteria (76.4%), of which respiratory rate (85.1%) and heart rate (74.5%) were the most common criteria. Most of the re-attendances were one-time (90.3%), and 30% developed sepsis and septic shock and required inpatient and critical care (45.8%). The unplanned re-attendance incidence was 16%, with an incidence rate of 6 persons per 1,000 persons/day. Middle-level hospitals had a higher re-attendance incidence (24%) than high-level (14.3%) and first-level (12.7%) hospitals. Factors affecting unplanned re-attendance to the ED within 30 days included comorbidities (hazard ratio [HR] = 5.0, 95% confidence interval [CI] = 3.056-8.413, p <
0.001), alcohol use (HR = 6.2, 95% CI = 3.555-10.854, p <
0.001), the model of care in the ED and discharge (HR = 11.1, 95% CI = 2.619-47.499, p <
0.001), and care in the ED and observed symptoms and discharge (HR = 13.8, 95% CI = 3.401-56.167, p <
0.001), which was the highest risk factor for unplanned re-attendance. CONCLUSIONS: The findings of this study indicate a high re-attendance occurrence among patients with SIRS. Both individual characteristics and health service delivery system efficiency play significant roles in influencing unplanned re-attendance. These factors can serve as valuable inputs for crafting policies aimed at enhancing the standard of care provided in EDs and guiding future research endeavors.