A 48-year-old male presents with diarrhea for 2 days, passing loose yellow stools with small amounts of mucus 10+ times daily, accompanied by mild left lower abdominal pain and tenesmus. Physical examination reveals only mild left lower abdominal tenderness and active bowel sounds. Suspected acute bacillary dysentery (ordinary type) is confirmed, and the patient is hospitalized for isolation. What is the criterion for discontinuing isolation?