There are many possible causes for bloody stool, including:[1][3]
Infection
Clostridioides difficile colitis
Shigella spp
Salmonella spp (i.e., nontyphoidal salmonella or enteritidis)
Campylobacter jejuni infection
Escherichia coli O157:H7 and other enterohemorrhagic Escherichia coli (EHEC) infections[4][5]
Viral gastroenteritis, usually caused by rotavirus or norovirus[6]
Parasitic infections, including Entamoeba histolytica and Giardia lamblia
Hookworm infection
Infection by Cryptosporidium or Cyclospora or Isospora belli or Microsporidia[6]
Bacterial (e.g., tuberculosis) or fungal infections (e.g., histoplasmosis, cryptococcosis, coccidioidomycosis, candidiasis)
Inflammatation such as:[7]
Inflammatory bowel disease
Ischemic bowel disease
Radiation colitis/proctitis
Ulcerative colitis
Diverticulitis
Infectious colitis including amebic colitis, pseudomembranous colitis and viral hemorrhagic colitis
Colorectal cancer
Trauma from rectal foreign body insertion or anal fissure
Bleeding disorders such as vitamin K deficiency
Meckel's diverticulum
Other causes (such as ingestion of iron pills, beets, or red dye)[1]
Upper gastrointestinal bleeding (if blood has a coffee ground-like appearance) including:[8]
Esophageal varices (rupture)
Peptic ulcer
Esophagitis
Mallory-Weiss tear
Cancer of the esophagus, stomach or pancreatic cancer and other upper gastrointestinal malignancies
Vasculitis
Aortic dissection
Bleeding disorders
Other (such as trauma, NSAID use, or angiodysplasia)[8]
Hemorrhoids
Anal fissures
The management and prognosis for people who present with bloody stool depends on the underlying cause.[9][10]
In case of lower gastrointestinal bleeding or hematochezia, there are 2 main differential diagnostic questions:[2]
Is it bleeding of small intestinal origin or of colonic origin?
What is the cause of the bleeding?
Physical examination for signs of hypotension and shock is important (to estimate blood loss). Vital signs (i.e. pulse, blood pressure, respiratory rate, and temperature) are also important. It is important to obtain a proper medical history, perform a physical examination and obtain laboratory studies (e.g., blood tests) to provide a better differential diagnosis and prognosis as different conditions have widely varying prognoses.[2] In a stable adult with lower GI bleeding, colonoscopy is indicated within 24 hours to allow evaluation of the entire colon and possible treatment of the source of bleeding using endoscopic therapy.[11] An emergency or urgent consultation with a physician is recommended if you are experiencing unexplained rectal bleeding for any reason.