Melena is a term that describes a particular appearance of blood in stools. This term describes stools that are black, tarry and often foul-smelling because of the presence of blood from the upper gastrointestinal tract. Melena is associated with gastrointestinal hemorrhage, peptic ulcers, gastritis and other upper gastrointestinal tract disorders.
The blood associated with melena gives stools a black, tarry appearance because blood is rich in a molecule called hemoglobin. This molecule is responsible for transporting oxygen through the bloodstream, and each molecule of hemoglobin contains an iron atom. As hemoglobin passes through the stomach and gastrointestinal tract, the iron goes through a chemical reaction called oxidation, which changes the color and appearance of the blood and stool.
Blood generally passes through the intestines quickly. When blood originates in the lower gastrointestinal tract, it typically is bright red, because the iron has not yet had time to oxidize. This means the source of oxidized blood usually is the upper gastrointestinal tract. Bright red or maroon-colored blood in stools is called hematochezia and usually originates from the large bowel, rectum or anus. This type of bleeding is most often caused by hemorrhoids. In rare cases, massive, rapid-onset bleeding in the stomach also can lead to bright-red blood in stools.
If one observes blood in the stool, a doctor should be consulted as soon as possible. The doctor will first ask for a medical history, which might include questions relating to medications being used, diet, gastrointestinal health and the presence of other symptoms in addition to blood in stools. The doctor might also perform a physical examination of the abdomen and rectum to determine whether any physical signs are present that might provide diagnostic clues.
Diagnostic tests that might be performed include blood tests, stool cultures and X-rays. Testing the stools is important because in certain specific situations, stools can take on the appearance of melena even if no bleeding is present. For example, eating black licorice, eating blueberries or taking iron pills can cause stools to have a similar appearance. Eating tomatoes or beets on a regular basis also can make stools appear red. The diagnostic process might also include a colonoscopy, a test that allows a doctor to examine the digestive tract closely to scan for growths.
People who experience one or more episodes of melena might be advised to modify their diet to prevent further bleeding, depending on the cause. For example, a diet high in natural fiber and low in saturated fat and alcohol might be recommended to reduce constipation and hemorrhoids. Medication might be prescribed if the bleeding has been caused by an ulcer. Some types of medications, such as over-the-counter anti-inflammatory drugs, can cause upper gastrointestinal bleeding, so people who often use these medications might be encouraged to reduce their intake.