A 32-year-old man sustains blunt abdominal trauma in a motor vehicle collision. On examination, he is haemodynamically stable with a heart rate of 88 bpm and blood pressure 128/82 mmHg. FAST examination reveals free fluid in the pelvis. CT abdomen with IV contrast shows a splenic laceration (Grade III) with active extravasation and a small amount of free fluid. Which finding best discriminates between a patient who requires immediate laparotomy versus one suitable for non-operative management with close monitoring?