A 68-year-old woman presents with fever, rigors, hypotension and confusion. She has a history of diabetes and a recent urinary tract infection. In the emergency department her blood pressure is 88/54 mmHg, heart rate 120 beats per minute, respiratory rate 26 breaths per minute and oxygen saturation 94% on room air. Lactate is 4.0 mmol/L. According to the Australian Sepsis Clinical Care Standard, what is the most critical time-sensitive action?