Blood in the stool (hematochezia, melena; ICD-10-GM K92.1: melena) is either red or black, depending on the length of time it has remained in the intestine or whether or not it has come into contact with gastric acid.
If red blood admixtures are detected in the stool, it is called hematochezia (synonyms: blood stool, rectal blood, rectal bleeding). The source of bleeding is often in the mid to lower gastrointestinal (GI) tract.Bright red fresh blood tends to indicate a source of bleeding in the distal colon, rectum, or anal canal. Dark clotted blood or coagulum is usually from the colon, often from the sigmoid (sigmoid colon) or proximal colon.
If the stool has a tar-like color, it is called melena (synonyms: tarry stools, melaena; ICD-10 K92.1: melena). The black color is caused by the content of hematin. This is formed when hemoglobin comes into contact with gastric acid. The color change is due to oxidation of iron in hemoglobin. The source of bleeding in a melena is usually located in the upper gastrointestinal tract (OGIB, upper gastrointestinal bleeding)/gastrointestinal tract. Most commonly, it is a hemorrhage above the papilla duodeni major (small bump containing the sphincter Oddii muscle, at the common orifice of the ductus choledochus (common bile duct) and ductus pancreaticus (pancreatic duct) into the duodenum (duodenum)), i.e., stomach or duodenum is most commonly involved. Less commonly, bleeding in the small intestine or ascending colon is the cause of melena. The tar-like color in bleeding in the small intestine or ascending colon is caused by bacterial decomposition of hemoglobin.
If blood is present in the stool without being visible to the eye, it is called occult blood (hidden blood).
Melena or hematochezia can be a symptom of many diseases (see under “Differential diagnoses”).
Course and prognosis: In the case of blood in the stool or accumulated blood (e.g., if hemorrhoids cause the bleeding), a physician should be consulted in any case. It is important to find the source of bleeding as soon as possible. This is particularly important in cases of severe bleeding (see “Gastrointestinal bleeding“). The course and prognosis depend on the cause as well as the degree of blood loss sustained.
