A 72-year-old man is brought to the Emergency Department with acute confusion, fever (38.5°C), and a productive cough for two days. His respiratory rate is 28/min, heart rate is 110/min, and blood pressure is 95/60 mmHg. His oxygen saturation is 90% on room air. According to the NICE Sepsis guidelines (NG51), what is the most important immediate action?