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Introduction: Traumatic extracranial carotid or vertebral artery injury (TCVI) occurs in about 1% of all blunt trauma admissions, carries a 10% risk of thromboembolic ischemic stroke, and accounts for up to 2,400 strokes annually in the U.S. Screening CT angiography (CTA) of trauma patients is ubiquitous, and patients with initially asymptomatic TCVI are commonly treated with antiplatelet agents to prevent stroke. Although prophylaxis with these agents is thought to be safer than anticoagulation, which carries a sgnificant risk of hemorrhage in trauma patients, their risk of hemorrhagic complications is unknown.Methods: Single-center retrospective cohort study of patients with a screening CTA after admission for blunt trauma. Patients with CTA indicating TCVI were treated routinely with aspirin 325 mg daily. Risk of transfusion ≥1 unit of packed red blood cells (PRBC) >24 hours after admission (excluding transfusions during injury-related blood loss) was assessed according to CTA findings (CTA+ or CTA-) and whether the patient was treated with aspirin (ASA+ or ASA-). Cox proportional hazards regression assuming equal time at risk was used to estimate risk ratios (RRs) and the relative excess risk due to interaction (RERI) for comparing risk of transfusion among the CTA/ASA groups.Results: The mean overall number of units of PRBC was 0.9±2.1 for CTA+/ASA+ patients (n=196) and 0.3±1.60 for CTA-/ASA- patients (n=2290) (<0.0001). When adjusting for age, ISS, and INR ≥1.2 at admission, the overall RR of transfusion was 1.81 (95% CI 1.40-2.35) for CTA+/ASA+ patients compared to CTA-/ASA- patients. Among age groups, CTA+/ASA+ patients ages 50-69 years had the greatest RR (1.91 CI [1.21-3.03]) compared to CTA-/ASA- patients.Conclusion: Use of aspirin to prevent stroke in patients with TCVI carries a significantly increased risk of PRBC transfusion. Future studies are needed to determine if this increased risk of transfusion is offset by a reduced risk of ischemic stroke.