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Background: Whether patients with acute stroke and large vessel occlusion (LVO) may benefit from prehospital identification and diversion by EMS to a center offering endovascular therapy (EST) is controversial. The accuracy of prehospital scales as predictors of LVO is only one feature of whether EMS can effectively identify patients suitable for EST. Other factors include accuracy of last known well time (LTKW) and identification of stroke mimics. We performed a population-based estimate of potential accuracy of field based identification of potential EST candidates in a large community setting.Methods: In Kaiser Permanente Northern California, all acute stroke patients arriving at its 19 primary stroke centers (PSC) between 7am and 12am were evaluated on arrival by stroke neurologists by video. We reviewed the teleneurology notes to determine the potential accuracy of EST selection based on NIHSS score > 7.Results: For 2016, there were 2546 total potential stroke alerts triaged by EMS as having potential acute strokes [Figure]. Of these, 1268 (50%) were not candidates for acute stroke treatment for various reasons including stroke mimics and inaccurate LTKW. Out of 1241 cases deemed candidates for acute stroke treatment, 638 (25.1%) had potential LVO based on NIHSS > 7. Of these, 116 (4.6% of total “potential strokes” and 18.2% of patients who had “severe” strokes) were diagnosed with LVO and treated with EST.Conclusions: Even if field based tools were as accurate as clinical scoring by stroke neurologists, less than 1 in 4 patients diverted to endovascular stroke centers and away from closer PSC would benefit by receiving EST. Given that 50% of patients triaged by EMS did not qualify for any acute stroke treatment, a lower percentage of patients would actually benefit from field based diversion. Stroke systems may be better served by focusing on rapid treatment, evaluation, and transfer to endovascular centers than field based diversion strategies.