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When referring to human feces, blood in stool looks different depending on how early it enters the digestive tract — and thus how much digestive action it has been exposed to — and how much there is. The term can refer either to melena, with a black appearance, typically originating from upper gastrointestinal bleeding; or to hematochezia, with a red color, typically originating from lower gastrointestinal bleeding. Evaluation of the blood found in stool depends on its characteristics, in terms of color, quantity and other features, which can point to its source, however, more serious conditions can present with a mixed picture, or with the form of bleeding that is found in another section of the tract .The term "blood in stool" is usually only used to describe visible blood, and not fecal occult blood, which is found only after physical examination and chemical laboratory testing.

In infants, the Apt test can be used to distinguish fetal hemoglobin from maternal blood based on the differences in composition of fetal hemoglobin as compared to the hemoglobin found in adults.

Evaluation
The tests that are considered for evaluation of the passage of blood in the stool is based on characteristic of bleeding (color, quantity) and whether or not the person passing blood has a low blood pressure with elevated heart rate, as opposed to normal vital signs. These tests are combined to determine which of the causes below is the source of bleeding.
 * Digital rectal exam (DRE) and fecal occult blood test (FOBT)
 * Colonoscopy
 * Anoscopy
 * EGD
 * Capsule endoscopy
 * CT Scan

Melena is defined as dark, tarry stools, often black in color due to partial digestion of the RBCs.

Hematochezia is defined as bright red blood seen in the toilet either inside of, or surrounding the stool

Hematochezia is typically presumed to come from the lower portion of the GI tract, and the initial steps of diagnosis include a DRE with FOBT, which if positive, will lead to a colonoscopy. If bleeding is large in volume, the evaluation may also include an EGD. If no source of active bleeding is found on these examinations, a capsule endoscopy may be performed, in order to more closely examine the small bowel, which cannot be seen with the other types of studies. With melena, a DRE with FOBT is often also performed, however the suspicion for a source from the upper GI tract is higher, leading first to the use of EGD with the other tests utilized if no source is identified. The anoscopy is another examination, which can be used as an adjunct to the colonoscopy, which exams the rectum and distal portion of the descending colon.

Other features
Mucus may also be found in stool.

A texture described as tarry stool is generally associated with dark black stool seen in partially digested blood. This is generally associated with melena.

Patient Age
Age is an important indicator for the diagnostic workup that is considered for the presence of blood in stool, as a result of the diagnoses that tend to affect each age group.

Differential diagnoses and pathophysiology
Blood in the stool can come from many sources. The causes range from not harmful to very serious conditions. A common way to divide causes of bleeding is based on the source of bleeding. The GI tract can be divided into upper and lower, with some causes of bleeding affecting the entire tract. Blood in the stool often appears different depending on its source, guiding the diagnostic approach of these conditions, however, depending on the rate of bleeding, it can appear different from the typical presentation of the source.

Upper GI tract
The upper GI tract is defined as the organs involved in digestion above the ligament of Trietz and is comprised of the esophagus, stomach, and duodenum. Upper gastrointestinal bleeding is typically characterized by melena, however bright red blood can be seen with active, rapid bleeding.

Lower GI tract
Bleeding of the lower GI tract will typically appear as hematochezia and can vary in degree of seriousness. Slow bleeding from the ascending portion of the colon can result in partial digestion of the blood and the appearance of melena in the stool.

Pathophysiology
The development of blood in stool results from a variety of conditions, which can be divided into major categories of disease process. These broad categories include cancerous processes or abnormal structure of bowel wall, inflammatory disease, colitis caused by infection or medications and vascular compromise.

Neoplasm

 * Colorectal cancer
 * Gastric cancer

Motility
The gut wall is important to the movement of waste products through the GI tract. Repetitive attempts to have a bowel movement can lead to tearing around the exit of the rectum (anal fissure),
 * Constipation

Structural
This list of diagnoses include diseases in which the wall of the bowel is compromised by disease.
 * Peptic ulcer disease - divided into either duodenal or gastric ulcers,  most common common causes include:
 * NSAID - the use of these medications results in a structural change in the wall of the gut, namely ulcers, and potential blood in the stool
 * H. pylori infection - this bacterial infection can erode the wall of the stomach or duodenum, leading to a structural change in the stomach wall and bleeding in the stool
 * Chronic disease


 * Diverticulosis
 * Meckel's diverticulum

Inflammatory bowel
Diseases causing inflammation in the GI tract can lead to blood in the stool as a result, which can occur anywhere along the GI tract in Crohn's disease or seen in the colon in ulcerative colitis.
 * Crohns disease
 * Ulcerative colitis

Colitis

 * Enteritis - inflammation of the small intestine, which may be caused by various forms of as well as by other conditions:

Infectious colitis

 * Food poisoning - the bacteria that is associated with bloody diarrhea is typically E. coli
 * Campylobacter enteritis
 * H. pylori infection
 * Shigellosis
 * Salmonellosis (salmonella enteritis/samonella enterocolitis)
 * Bacterial gastroenteritis
 * Campylobacter jejuni
 * Clostridium dificile
 * Escherichia coli enteritis - most common cause of travelers' diarrhea
 * Salmonella enterica
 * Shigella dysenteriae see also dysentery
 * Staphylococcus aureus

Drug-induced colitis

 * Radiation enteritis
 * NSAID induced peptic ulcer disease

Vascular compromise

 * Angiodysplasia of the GI tract
 * Arteriovenous malformation
 * Anal fissure is a tear in the skin and mucosa, they also result in ischemia of the blood vessels near the surface leading to prolonged healing
 * Esophageal varices
 * Hemorrhoids
 * Internal hemorrhoids are covered by a layer of mucosa and epithelium, making them more likely to bleed, but typically do not cause pain
 * External hemorrhoids are less likely to bleed, they are covered by a different type of epithelium (squamous) but can cause significant pain as a result of thrombosis of the blood vessels within them


 * Polypectomy during a colonoscopy can lead to a small amount of bleeding seen in the stool after the procedure

Treatment
Treatment of bloody stool depends largely on the cause of bleeding, however, bleeding is commonly associated with symptoms of fatigue, dizziness, headaches, or even shortness of breath, and these associated symptoms require treatment. This occurs due to a lack of RBCs circulating in the vascular system, resulting in less oxygen reaching the tissues and organs, leading to the symptoms listed. Blood in stool can be associated with serious complications as a result of blood volume loss (hemorrhage) or a slow leak of the blood leading to low levels of hemoglobin in the circulating blood (anemia).

Anemia
A common complication of blood in the stool, especially when voluminous, or occurring over a long period of time, is anemia.

Anemia is also commonly associated with iron deficiency, due to the importance of iron in the formation of red blood cells. Other vitamins associated with the RBC formation, and thus the treatment of anemia are folate, vitamin B12 and vitamin C. When anemia is diagnosed as a result of blood in the stool, these vitamins are frequently prescribed in order to ensure that all the materials are available for those cells that are made.

Specific Treatment
Diagnostic measures can also be used as interventions to help stop bleeding in some cases. Bleeding as a result of neoplasm can be treated using colonoscopy and clipping, surgical intervention, or other measures, depending on the form and stage of cancer. Similarly, gastric cancer is treated depending on the staging, although typically requires surgical and medical therapy.

The treatment for motility issues, namely constipation, is typically to improve the movement of waste though the GI tract, by use of stool softeners (which work by pulling water into the stool while in the colon), addition of fiber to the diet, and use of osmotic laxatives (which facilitate fluid movement through the colon, improving overall motility), thus reducing the straining during defection and decreasing the development of anal fissures. Anal fissures commonly are associated with pain and blood on the tissue paper, and require time for healing so that ischemic blood vessels become perfused again. Treatment includes topical nitrates or calcium channel blockers as well as surgical intervention. Similar to anal fissures, internal hemorrhoids can cause blood on the tissue when wiping, and be felt at the opening of the anus. Treatment options for hemorrhoids can be dependent on whether an underlying cause exists, such as anorectal varices related hemorrhoids caused by cirrhosis, however symptomatic treatment often involves removal.

As stated above, colitis can be divided into infectious and drug induced, as can the treatment for these conditions. With infectious colitis, treatment is pathogen dependent, and generally requires the use of antibiotics. With drug-induced colitis, treatment typically involves removal of the offending agent, as is the case in NSAID induced PUD, however, removing radiation from a cancer patient is not always practical within a treatment regimen, so medical treatment is the primary mode of treatment.

Structural compromise leading to blood in stool is caused by a variety of conditions, and therefore requires different treatment for each condition. Peptic ulcer disease alone can be divided into multiple causes, but is generally initially controlled primarily with a proton pump inhibitor, with the addition of an H2 blocker, or in serious cases, requiring surgical intervention. Diverticulitis and diverticulosis require antibiotic treatment, and may require surgical intervention.

Inflammatory bowel disease is also divided into separate conditions, namely ulcerative colitis and Crohn's disease, which have different medical treatment regimens, and may require surgical intervention in more serious conditions.

Treatment of blood in stool is largely dependent on the source and seriousness of the condition, and a variety of treatment options are available.

Sources to use in medical articles include: practice guidelines and review articles (no primary sources)