cancers, vascular disorders, adverse effects of medications, and blood clotting disorders. Obscure gastrointestinal bleeding (OGIB) is when a source is unclear following investigation. Upper gastrointestinal Upper gastrointestinal bleeding is from a source between the pharynx and the ligament of Treitz. An upper source is characterised by hematemesis (vomiting up blood) and melena (tarry stool containing altered blood). About half of cases are due to peptic ulcer disease (gastric or duodenal ulcers). Esophageal inflammation and erosive disease are the next most common causes. In those with liver cirrhosis, 50–60% of bleeding is due to esophageal varices. Approximately half of those with peptic ulcers have an H. pylori infection. Other causes include Mallory-Weiss tears, cancer, and angiodysplasia. A number of medications are found to cause upper GI bleeds. NSAIDs or COX-2 inhibitors increase the risk about fourfold. SSRIs, corticosteroids, and anticoagulants may also increase the risk. The risk with dabigatran is 30% greater than that with warfarin. Lower gastrointestinal Lower gastrointestinal bleeding is typically from the colon, rectum or anus. Common causes of lower gastrointestinal bleeding include hemorrhoids, cancer, angiodysplasia, ulcerative colitis, Crohn's disease, and aortoenteric fistula. It may be indicated by the passage of fresh red blood rectally, especially in the absence of bloody vomiting. Lower gastrointestinal bleeding could also lead to melena if the bleeding occurs in the small intestine or proximal colon. Signs and symptoms Gastrointestinal bleeding can range from small non-visible amounts, which are only detected by laboratory testing, to massive bleeding where bright red blood is passed and shock develops. Rapid bleeding may cause syncope. The presence of bright red blood in stool, known as hematochezia, typically indicates lower gastrointestinal bleeding. Digested blood from the upper gastrointestinal tract may appear black rather than red, resulting in "coffee ground" vomit or melena. Other signs and symptoms include feeling tired, dizziness, and pale skin color. A number of foods and medications can turn the stool either red or black in the absence of bleeding. Bismuth found in many antacids may turn stools black as may activated charcoal. Blood from the vagina or urinary tract may also be confused with blood in the stool. Diagnosis Diagnosis is often based on direct observation of blood in the stool or vomit. Although fecal occult blood testing has been used in an emergency setting, this use is not recommended as the test has only been validated for colon cancer screening. Differentiating between upper