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. 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.
Location of Bleed
Melena typically indicates bleeding in the upper gut – esophagus, stomach and first parts of the small intestine. The bleeding is from the walls of the gut and due to the action of air and digestive enzymes, the blood degrades to appear black in color often with a slightly foul odor. Many people may not notice this change in the stool unless there is significant blood loss in the gut.
Hematochezia indicates bleeding in the lower gut – ileum of the small intestine, colon, rectum or anus. Often small amounts of blood in the stool is not easily noticed. In many cases bleeding is so minor that it requires special tests to identify. However, when bleeding is significant then then the blood in the stool is bright red and still fluid and therefore easily evident when wiping.
What is melena?
Melena is the term used to describe black tarry feces which is a result of “old” and degraded blood in the stool. It usually indicates an upper gastrointestinal bleed originating within the esophagus, stomach or duodenum of the small intestine.
Less frequently, bleeding from the mouth or nose may also be responsible but this is more clearly evident when vomiting (hematemesis), coughing (hemoptysis) and in the nasal discharge (epistaxis) or sputum (saliva, phlegm). If there is slow transit time (GI motility), the bleed could be from lower down the gut which typically presents with fresh blood in the stool (hematochezia).
As little as 50 ml of blood in the upper gut can result in melena although it usually requires 100 to 200ml before it becomes clearly evident. While it is reported that melena results in foul-smelling stool, this is sometimes difficult to differentiate from the normal odor. The smell of “old” blood emanating from the stool may accompany the black tarry appearance of the feces.
If there are any signs of shock, the condition needs to be treated as a medical emergency.
Causes of Dark Blood in the Stool
These conditions may arise secondary to the contributing factors listed below.
- Esophageal cancer
- Bleeding stomach ulcer (refer to Causes of Stomach Bleeding)
- Bleeding duodenal ulcer
- Varices of the esophagus or stomach
- Mallory-Weiss tear
- Tumors of the gastrointestinal tract
- Arteriovenous malformation
- Meckel’s diverticulum
- Gastroesophageal reflux disease (GERD)
- Liver cirrhosis
- Certain drugs that are known to cause an upper GI bleed like aspirin and other NSAID’s
- Trauma to the gastrointestinal tract, which includes investigative medical procedures like an upper GI endoscopy and surgery
- Foreign body lodged in the gut or passing through the gut
- Gut infections like H.pylori (stomach) or the various causes of infectious gastroenteritis (stomach and duodenum). Esophageal infections due to candida or herpes may be seen in immunocompromised patients like in HIV/AIDS
- Generalized infections like typhoid fever and yellow fever. Rare infections include the Ebola virus and icteric leptospirosis (Weil disease)
- Venom from poisonous insects, arachnids, reptiles
- Metals like in arsenic poisoning
- Pest and rodent poisons
- Drug (ingested) overdose
- Alcohol poisoning
- Bleeding disorders like hemophilia
What is hematochezia?
Hematochezia is the medical term for the presence of ‘fresh’ blood in the stool. This may vary from gross bleeding in the stool or with a bowel movement which appears red to maroon in color. Red strands or streaks in the stool or red to pink staining of the toilet bowl water may also be an indication of hematochezia. T
he stool may be coated with blood, only visible upon wiping or the blood may be within the stool and therefore not clearly evident. Sometimes the blood is not visible to the naked eye due to the small quantity (occult bleed) and is only detected upon testing.
Hematochezia, in contrast to melena, indicates a recent bleed in that the blood has not been exposed to oxygen, digestive enzymes or intestinal bacteria for a prolonged period so as to degrade the blood. In most cases, this is indicative of a bleed in the lower gut – namely the colon, rectum or anus.
However, excessive bleeding in the upper gut – esophagus, stomach and first half of the small intestine – or upper GI bleeding in the event of rapid transit time, as with diarrhea, may result in hematochezia. Similarly, bleeding in the ascending colon can result in melena, especially if there is impaired colonic motility, large bowel obstruction or constipation.
Causes of Fresh Blood in the Stool
The causes and/or risk factors of the more common conditions leading to hematochezia are also listed.
- Sitting on the toilet for long periods
- Straining during bowel movements
- Persistent diarrhea or chronic constipation
- Over 40 years of age
- Obesity and sedentary lifestyle
- Low dietary fiber
- Anal fissure
- Large stool and/or hard stool
- Straining during bowel movements
- Persistent diarrhea
- Crohn’s disease
- Bleeding anal rash
- Any excoriated skin rash around the anus, including rashes due to sexually transmitted diseases (STD’s), contact or atopic dermatitis, diaper rash, xerosis (dry skin), or psoriasis.
- Trauma or Foreign bodies
- Incorrect use of suppositories
- Home enemas and colon cleansing therapies (unskilled therapist)
- Anal intercourse
- Colonoscopy at the hands of an unskilled practitioner
- Colorectal polyps
- Colorectal cancer
- Anal cancer
- Inflammatory bowel disease
- Ischemic colitis
- Infectious enterocolitis
- Peri-anal abscess
- Vascular malformation