Introduction: Acute aortic syndrome (AAS) is a rare clinical syndrome with a high mortality encompassing acute aortic dissection, intramural hematoma and penetrating atherosclerotic ulcer. Up to 38% of cases are misdiagnosed on first presentation. There is a large variation in use of computed tomography to rule out AAS. The Canadian clinical practice guideline for the diagnosis of AAS was developed in order to reduce the frequency of misdiagnoses. As part of the guideline, a clinical decision aid was developed to facilitate clinician decision-making based on practice recommendations. Our objective was to validate the sensitivity of this clinical decision aid. Methods: Our validation cohort was recruited from a retrospective review of all cases of AAS diagnosed at three tertiary care emergency departments and one cardiac referral center from 2002-2019. Inclusion criteria: >18 years old, non-traumatic, symptoms <14 days and AAS confirmed on computed tomography, transesophageal echocardiography, intraoperatively or postmortem. The clinical decision aid assigns an overall score of 0-7 based on high risk pain features, risk factors, physical examination and clinical suspicion. Sensitivity with 95% confidence intervals are reported. Based on a national survey, a miss rate of <1% was predefined for the validation threshold. Results: Data was collected from 2002-2019 yielding 222 cases of AAS (mean age of 65 (SD 14.1) and 66.7% male). Kappa for data abstraction was 0.9. Of the 222 cases of AAS (type A = 125, type B = 95, IMH = 2), 35 (15.7%) were missed on initial assessment. Patients were risk stratified into low (score = 0, 2 (0.9%)) moderate (score = 1, 42 (18.9%)) and high risk (score ≥2,178 (80.2%)) groups. A score ≥1 had a sensitivity of 99.1% (95% CI 96.8-99.9%) in the detection of AAS. The clinical decision aid missed 0.9% (95% CI 0.3-3%) of cases. Conclusion: The Canadian clinical practice guideline's AAS clinical decision aid is a highly sensitive tool that uses readily available clinical information. Although the miss rate was <1%, the 95% confidence intervals crossed the predefined threshold. Further validation is needed in a larger population to ensure the miss rate is below an acceptable level.