A 40-year-old man is admitted to ICU following an out-of-hospital cardiac arrest with ROSC. He remains comatose (GCS 3T) at 72 hours post-arrest. The team is assessing neurological prognosis. EEG shows a continuous, reactive background pattern. Bilaterally absent pupillary light reflexes are noted. Serum neuron-specific enolase (NSE) at 72 hours is 35 µg/L. What is the most reliable predictor of poor neurological outcome?