A 70-year-old man presents with fever, rigors and flank pain. He is hypotensive (BP 86/50 mmHg), tachycardic and confused. Urinalysis shows nitrites and leukocytes. Sepsis due to pyelonephritis is suspected. After taking cultures and giving broad-spectrum intravenous antibiotics, what is the most important next step in management according to sepsis principles?