Melena - 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 (like Hematochezia) is often 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.
False Malena - black stool NOT caused by blood can be due to:
Blood that originates from a high source in the digestive tract or bleeding from a lower source that occurs slowly enough to allow for
oxidation, is associated with tarry poop melena.
For this reason, black melena stool is often associated with blood in the stomach or the upper gastrointestinal tract. It takes approximately 14 hours for blood to be broken down within the intestines and go from red to black color. Therefore, if transit time is less than 14 hours the patient will have hematochezia instead of black or tarry melena, and if greater than 14 hours, melena.
One rule of thumb is that black and tarry poop (melena) only occurs if the source of bleeding is above the ligament of Treitz although frequent exceptions occur.
The most common cause of (black tarry poop) 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 the bleeding surface of them.