Blood in the stool originates from one thing: bleeding into the gastrointestinal tract. The bleed may be slow and chronic or swift and of sudden onset. In diagnosing the cause of blood in the stool the first step is to find the source of the bleed or, in other words, to answer where exactly it is coming from. The second step is to diagnose the nature of the lesion.
Anatomy of GI Tract Bleeds
Generally GI bleeds are divided into upper GI bleeds and lower GI bleeds. The upper portion of the GI tract extends from the mouth to the first part of the small intestine called the duodenum. The lower GI tract runs from this juncture to the anus. Bleeds in the upper GI tract generally cause hematemesis, or bloody vomit. However, they will cause blood in the stools if the bleeding is brisk. Bleeding in the lower GI tract usually leads to blood in the stools, according to MedLine.
Causes of Upper GI Bleeds
Common causes of esophageal bleeding include small tears in the esophagus from forceful vomiting, called a Mallory Weiss tear or the rupture of dilated veins in the esophagus as a result of liver cirrhosis. In the stomach bleeding may be caused by ulcers or erosions from gastritis, which is inflammation of the stomach. Gastritis stems from excess stomach acid or is caused by medications or infections. Infrequently, upper GI bleeds may be caused by benign or cancerous tumors or other lesions to the blood vessels, according to the National Digestive Diseases Information Clearinghouse.
Presentation of Lower GI Bleeding
A 2005 study by Lisa Strate M.D., M.P.H. in Gastroenterology Clinics of North America indicated that 87 percent of blood in the stools originated in the lower GI tract. Bleeding farther up in the GI tract tends to make melena, which is described as black tarry stools. Bleeding lower in the GI tract causes hematochezia, which is blood mixed in the stools. Brisk bleeding or bleeding close to the anus may present as bright red blood. In addition, blood may be found on routine stool testing by a health care provider.
Causes of Lower GI Bleeds
The most common reason for a lower GI bleed is diverticulosis, which is a sac-like out-pouching of the large intestinal wall, exposing blood vessels to the GI tract. Another cause is angiodysplasia---dilated tortuous blood vessels. Infections such as food poisoning may also cause blood in the stools. Inflammatory bowel disease is another cause, including Crohn's disease and ulcerative colitis. Colon cancer is less common but also may present with a GI bleed. Hemorrhoids and anal fissures cause bleeding as well, particularly in younger patients.
Considerations
Upper GI bleeds may be mimicked by nose bleeds, as the blood may be swallowed and then vomited. Red foods may also be falsely mistaken for blood. According to EB Medicine, lower GI bleeds may also be mimicked by bismuth containing medications such as Pepto-bismol, activated charcoal, beets, red-grapes, vaginal bleeding and other external lesions. False positive blood tests may be caused by red meat, turnips, horseradish and vitamin C.
References
- Gastrointestinal Bleeding: MedlinePlus Medical Encyclopedia
- Bleeding in the Digestive Tract
- "Gastroenterology Clinics of North America"Lower GI bleeding: epidemiology and diagnosis; Strate; 2005
- EB Medicine. Gastrointestinal Bleeding: An Evidence-Based ED Approach To Risk Stratification
