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To date, the clinical application of near infrared spectroscopy in the adult brain has been limited. The NIRO 300 (Hamamatsu Photonics) provides a continuous measurement of tissue oxygen index (TOI) using spatially resolved spectroscopy. Although TOI reflects cerebral oxygenation to a high degree of sensitivity and specificity, to become a useful clinical tool, thresholds for cerebral ischemia need to be defined. This study has attempted to identify a quantifiable TOI threshold for ischemia in the adult brain.One hundred sixty-seven patients undergoing carotid endarterectomy were studied. The NIRO 300 was incorporated into an established multimodal monitoring system enabling observations of cerebral hemodynamic changes under highly controlled conditions. Changes in TOI (ΔTOI) on clamping the internal carotid artery were compared with intracranial blood flow (middle cerebral artery flow velocity) and cerebral function monitoring to identify and quantify periods of cerebral ischemia.Significant correlation was seen between changes in middle cerebral artery flow velocity and ΔTOI on clamping (r=0.74, P=0.0001). Thirty-one patients showed cerebral ischemia on internal carotid artery clamping as defined by a sustained fall in cerebral function monitoring. A threshold for %ΔTOI of −13 was identified, above which no patients showed any evidence of ischemia on clamping. This threshold provided 100% sensitivity and 93.2% specificity for patients satisfying the preset criteria for cerebral ischemia.These data demonstrate the potential to identify TOI-quantified thresholds for cerebral ischemia in the adult brain and thus improve the clinical use of near infrared spectroscopy. Our observations have defined a drop in TOI (13%) that can be adopted as a threshold for severe cerebral ischemia with high sensitivity and specificity.