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Introduction: Patient selection for thrombectomy of acute ischemic stroke (AIS) caused by large vessel occlusion (LVO) in the delayed time window (>6 hours) is dependent on delineation of clinical-core mismatch or radiological target mismatch using perfusion imaging. Selection paradigms not involving advanced imaging and software processing may reduce time to treatment and broaden eligibility. We aim to develop a conversion factor to approximately determine the volume of hypoperfused tissue using the NIHSS score [CAT volume (clinically approximated tissue)] and explore its ability to identify patients eligible for thrombectomy in the late time window. Methods: We performed a retrospective analysis of anterior circulation LVO strokes at three comprehensive stroke centers. Demographic, clinical (NIHSS score, TLKW-time last known well) and imaging [computed tomography with perfusion (CTP) processed using RAPID, IschemaView] information was analyzed. A conversion factor, which is a multiple of the NIHSS score (one multiple for NIHSS score 6 seconds volume) was tested using DEFUSE-3 criteria eligibility as a gold standard. Result: Of the 309 LVO strokes [mean age of 70 ±14, 46% male, median NIHSS 16 (12-20)] included in this study, 38% of patients arrived beyond 6 hours of TLKW. Conversion factors derived (derivation cohort-center A:187) based on median (50 th percentile) values of Tmax >6s volume for NIHSS Conclusions: Clinical severity of stroke (NIHSS score) may be used to calculate the volume of hypoperfused tissue during LVO stroke. Clinically approximated hypoperfused tissue (CAT) volumes for NIHSS score