Blood coming out of the anus is called blood in the stool and is a common symptom of gastrointestinal and anal diseases. The color, nature and accompanying symptoms of blood in stool can help diagnose and treat the disease. Zhao Yongchang, Department of Anorectal Surgery, The First Affiliated Hospital of Guangzhou University of Traditional Chinese Medicine In general, bright red blood in stool indicates that the bleeding site is in the distal colon, rectum or anus; dark red blood in stool with mucus indicates that the bleeding site is in the proximal colon or small intestine; black stool or asphalt colored stool indicates that the blood in stool comes from the esophagus, stomach or duodenum; sometimes, blood in stool may also occur due to swallowing bleeding from the mouth, nose or trachea. Small amounts (microscopic) of GI bleeding do not cause a change in stool color, but only a positive fecal occult blood test, called occult blood stool. Of course, the color of blood in the stool may vary depending on the site of bleeding, the amount of bleeding, and the rate of bleeding. The initial diagnosis of the disease can be made based on the accompanying symptoms of blood in the stool: bright red blood in the stool with dry, hard stool and anal pain may be an anal fissure. Blood on hand paper or blood dripping from the anus after a bowel movement, with bright red blood and no anal pain, may be internal hemorrhoids. If the blood in the stool is dark red, foul-smelling or with pus, accompanied by an increase in the frequency of stool and a feeling of inability to pass the stool, or accompanied by thinning of the stool and weight loss, it may be a malignant rectal tumor. The occurrence of mucus and blood stools with no obvious cause in middle age or above, accompanied by abdominal distension, abdominal pain and change in stool habit, may be intestinal tumor. Sudden onset of bloody watery stools with abdominal pain and diarrhea, or with nausea and vomiting, fever, and a history of unclean diet before the onset, may be acute hemorrhagic necrotizing colitis. Recurrent diarrhea with mucopurulent or bloody stools, or with abdominal distension and abdominal pain, wasting, may be non-specific inflammatory bowel disease such as Crohn’s disease, ulcerative colitis. Paste-like or mucopurulent blood in the stool with right lower abdominal pain, low fever in the afternoon, night sweats, and emaciation may be intestinal tuberculosis. Bright red blood in stool without anal pain, abdominal pain and other discomfort in children may be intestinal polyps. Sudden onset of paroxysmal abdominal pain, red jam-like stools or with abdominal masses in infants and children may be intestinal trapped. Occult blood stools can occur in all diseases causing gastrointestinal bleeding, commonly ulcers, inflammation, and tumors. Regular fecal occult blood testing is an important way to screen for colorectal tumors (primary screening). There are many other causes of blood in stool, including bacterial dysentery, typhoid and paratyphoid fever, rheumatoid arthritis, etc. Oral administration of certain drugs such as adrenal corticosteroids and aspirin can induce bleeding of gastrointestinal mucosa damage or hemorrhagic erosive gastritis, which can cause black stool or blood in stool. Blood in the stool is a symptom of many diseases, so it should be taken seriously and go to the anorectal surgery department of the hospital in time for relevant examinations such as anal finger examination, anoscopy, colonoscopy, gastroscopy, etc. to clarify the cause and treat it actively.