A 38-year-old woman with acute appendicitis undergoes emergency surgery. Intraoperatively, a perforated appendix with peritoneal contamination is found. Postoperatively, she develops fever (38.5°C), tachycardia (110 bpm), tachypnea (22/min), and WBC 14,500/μL. Blood cultures are pending. Which single finding best distinguishes sepsis from uncomplicated systemic inflammatory response syndrome (SIRS) in this patient?