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In medicine, melena or melæna refers to the black, "tarry" feces that are associated with gastrointestinal hemorrhage. The black color is caused by oxidation of the iron in hemoglobin during its passage through the ileum and colon.
Melena vs. hematochezia 
Bleeds that originate from the lower GI tract (such as the sigmoid colon and rectum) are generally associated with the passage of bright red blood, or hematochezia, particularly when brisk. Only blood that originates from a high source (such as the small intestine), or bleeding from a lower source that occurs slowly enough to allow for enzymatic breakdown is associated with melena. For this reason, melena is often associated with blood in the stomach or duodenum (upper gastrointestinal tract), for example by a peptic ulcer. A rough estimate is that it takes about 14 hours for blood to be broken down within the intestinal lumen; therefore if transit time is less than 14 hours the patient will have hematochezia, and if greater than 14 hours the patient will exhibit melena. One often-stated rule of thumb is that melena only occurs if the source of bleeding is above the ligament of Treitz although, as noted below, exceptions occur with enough frequency to render it unreliable.
Patients sometimes present with anemia or low blood pressure. Very often, however, aside from the melena itself, there are no other symptoms. A digital rectal exam must be performed to assess for both presence and possible source of blood. Testing for blood in the stool with such tests as the stool guaiac are often performed, though it has only been validated for colorectal cancer screening. If this is to be performed, it should be done before the digital rectal exam to avoid trauma to the anal canal, and its ability to change management is debatable. If a source in the upper GI tract is suspected, an upper endoscopy can be performed to diagnose the cause.
Lower GI bleeding sources usually present with hematochezia or frank blood. A test with poor sensitivity/specificity that may detect the source of bleeding is the tagged red blood cell scan. This is especially used for slow bleeding (<0.5 ml/min). However, for rapid bleeding (>0.5 ml/min), mesenteric angiogram ± embolization is the gold standard. Colonoscopy is often first line, however.
The most common cause of melena is peptic ulcer disease. Any other cause of bleeding from the upper gastro-intestinal tract, or even the ascending colon, can also cause melena. Melena may also be a sign of drug overdose if a patient is taking anti-coagulants, such as warfarin. It is also caused by tumors, especially malignant tumors affecting the esophagus, more commonly the stomach & less commonly the small intestine due to their bleeding surface. However,the most prominent and helpful sign in these cases of malignant tumours is haematemesis. It may also accompany hemorrhagic blood diseases (e.g. purpura & hemophilia). Other medical causes of melena include bleeding ulcers, gastritis, esophageal varices, and Mallory-Weiss syndrome.
Causes of "false" melena include iron supplements, Pepto-Bismol, Maalox, and lead, blood swallowed as a result of a nose bleed (epistaxis), and blood ingested as part of the diet, as with consumption of black pudding (blood sausage), or with the traditional African Maasai diet, which includes much blood drained from cattle.
Melena is usually not a medical emergency because the bleeding is slow. Urgent care however is required to rule out serious causes and prevent potentially life-threatening emergencies.
A less serious, self-limiting case of melena can occur in newborns two to three days after delivery, due to swallowed maternal blood.
See also 
- MedlinePlus Encyclopedia Bloody or tarry stools, retrieved 19 July 2010.
- Navarra, Tova (2004). The Encyclopædia of Vitamins, Minerals, and Supplements. Infobase Publishing. "Iron supplements cause a grayish-black stool that should be distinguished from the black, tarlike stool that occurs from bleeding ulcer."