There are two medical terms that are used to describe the presence of blood in the stool.
- Melena (UK ~ melaena) is the passage of black, tarry stools which is indicative of blood that has degraded.
- Hematochezia is the passage of bloody stools, where the blood appears fresh and red to maroon in color.
Color of Blood in Stool
The normal color of stool varies from light to dark brown. This is due to the presence of bilirubin and the decomposition of food. If the stool is black or red, bleeding within the gastrointestinal tract needs to be excluded, especially if it is ongoing. Certain foods, drinks and drugs may cause discoloration of stool ranging from black to red or maroon. This discoloration may not be due to any bleeding. A small amount of blood in the stool is usually not visible and therefore goes unnoticed (occult) until it is detected upon a fecal occult blood test. The color of blood in the stool may vary depending on the site of the hemorrhage and transit time.
Melena is usually indicative of bleeding originating from higher up in the gut – esophagus, stomach and duodenum (first part of the small intestine). As blood passes down the gut to the rectum, it is subjected to the action of digestive enzymes, air (oxidation) and bacteria. This causes the blood cells to breakdown and changes the color to a dark maroon (burgundy) to black in color. Sometimes the bleeding may originate from the mouth or nasal cavities. A person with black tarry stools may also report the odor of “old” or “stale” blood emanating from the stool.
It takes at least 50ml of blood in the upper gut to result in black, tarry stools although it may require between 100 to 200 ml for melena to be clearly evident. Bleeding in the upper gastrointestinal tract may be accompanied by hematemesis, which is the sign of blood in the vomit or even hemoptysis which is the coughing up of blood. Melena may also be a sign of bleeding within the lower parts of the small intestine or less frequently in the colon if the blood has been present in the stool for a prolonged period of time, which may occur in a person who is constipated.
The various causes of melena are discussed further under Dark Blood in Stool.
Hematochezia is a sign of bleeding within the lower gut – large intestine, rectum and anus. However, rapid bleeding within the upper gastrointestinal tract may also cause hematochezia. Rectal bleeding and bleeding from the anus are the most common causes of hematochezia but it can arise from higher up the gut.
Hematochezia may be clearly evident in the stool, coating the feces, presenting as red strands/streaks or staining the toilet bowl. At other times, it may only be noticed upon wiping the anus. The blood may be encompassed within the stool and therefore not clearly evident. If the bleeding is associated with severe diarrhea, it will result in watery stools that appear pink to red in color.
The various causes of hematochezia are discussed further under Fresh Blood in Stool.