Background: As the population ages, the prevalence of atrial fibrillation (AF) is increasing. This leads to significant healthcare resource use – mainly the cost of hospitalization. Using guidelines, we built the Ambulatory Care Score for Atrial Fibrillation (ACSAF) for application in emergency departments (EDs) that lack a cardiologist. The primary objective of the present study was to validate the ACSAF for the triage of new-onset AF (NOAF) by comparison with the cardiologist’s opinion.Methods: This single-centre, prospective, observational, comparative study was performed at a general hospital between November 2018 and October 2019. All adult patients with NOAF were included. Patients with a systolic blood pressure <80 mmHg or a heart rate >180 bpm were excluded. The ACSAF was calculated by an ED physician in blind to the cardiologist’s opinion. The primary outcome was the mean difference in the ACSAF between ambulatory care group and the hospital care group. A receiver operating characteristic (ROC) curve was plotted to determine the optimal cut-off for the ACSAF. Results: 81 patients with NOAF (42 men (51.9%); mean age: 69 ±16 years old) were included in the study. The mean ACSAF was higher in the hospital care group (2.16 ±1.05) than in the ambulatory care group (1.03 ±0.97; p<0.0001). The optimal ACSAF cut-off was 2, giving a sensitivity [95% confidence interval] of 71.4%[57.1-83.7] and a specificity of 81.2%[68.8-93.8]. Discussion & Conclusion: The ACSAF enables ED physicians to decide on the triage of NOAF in the absence of a cardiologist’s opinion.Trial registration: ANSM (French Drug Administration) : ID RCB 2018-A01800-55 French National Data Protection Commission (Commission nationale de l'informatique et des libertés (Paris, France); reference: MRC-003 2197970, June 2nd, 2018)