Blood in Stool Causes and Fecal Occult Blood Test (FOBT)

Meaning of Blood in the Stool

When blood in the stool results from rough wiping or hemorrhoids, it is usually not dangerous. In most other cases, it may be a symptom of a serious disorder, and has to be checked by a doctor as soon as possible. In general, blood in the stool from the end of the gut (anus, rectum) is bright red and liquid, and blood from the stomach or above is black and thick. Massive bleeding from the stomach may still appear as red blood, and blood from the rectum may be dark, since colonic bacteria may change it color (in several hours).

What Does the Blood in the Stool Look Like?

The blood in the stool may appear as:

  • Invisible (occult)
  • Bright red fluid
  • Red streaks on the stool surface
  • Red or maroon colored stool
  • Black tarry stool

Bright Red Liquid Blood in the Stool

Red blood in the stool, medically called hematochezia, can result from:

  • Rough wiping
  • Injury during passing hard stool (in constipation)
  • Menstruation
  • External or internal hemorrhoids
  • Anal fissure
  • Foreign body or injury
  • Diverticulitis
  • Colonoscopy
  • Polyp or cancer in the rectum or colon
  • Crohn’s disease or ulcerative colitis
  • Amebiasis
  • Shigellosis
  • Intestinal worms
  • Massive bleeding from small intestine
  • Rupture of the small intestine or colon due to cancer, ulcer, or surgical procedure
  • Bleeding from the urinary tract
  • Fistula between the urinary bladder and the rectum
  • Spread of prostatic cancer into the rectum
  • Portal hypertension (in liver cirrhosis)
  • Vitamin K deficiency
  • Vascular malformation
  • Trombocytopenia
  • Amyloidosis

Red Streaks, Red or Maroon Stool

Read streaks on the surface of the stool may appear in most of above mentioned causes (if bleeding is slow), or in:

  • Duodenal ulcer or cancer
  • Ischemic colitis (due to abdominal atherosclerosis)
  • Microscopic colitis
  • Appendicitis
  • Celiac disease
  • Intestinal worms
  • Small intestinal polyps or lymphoma
  • Meckel’s diverticulum ( a patch of gastric (stomach) mucosa that appear in the small intestine, secretes acid, causes ulcers, and may bleed)
  • Bowel obstruction (ileus, intussusception)
  • Poisons: arsenic, warfarin, lead
  • Pancreatic cancer
  • Cancer of biliary duct
  • Radiation therapy
  • Heavy metal (lead) poisoning
  • Necrotizing enteritis
  • Typhoid fever, yellow fever
  • Drugs (e.g. Methacycline)
  • Hemolytic-uremic syndrome
  • Toxic megacolon
  • Neurofibromatosis
  • Wiskott-Aldrich syndrome

Normal bowel movement color

Bloody Diarrhea

Bloody diarrhea, or diarrhea with a hidden blood may appear in:

  • Heavy bacterial or parasitic infection (food or water poisoning)
  • Inflammatory bowel disease
  • Diverticulitis
  • Other causes of bloody diarrhea are rare (Ebola virus…)

Black, Tarry Stool

Black, tarry stool, medically called melena, that sticks to the toilet, and is foul smelling, may result from:

  • Stomach inflammation (gastritis), erosion, ulcer, or cancer
  • Duodenal ulcer or cancer
  • Pancreatic or biliary duct cancer
  • Meckel’s diverticulum
  • Esophageal varices
  • Mallory-Weiss syndrome (tear of esophageal mucosa during violent vomiting)
  • Extracted tooth, injury, heavy inflammation, or cancer in the mouth, throat, or esophagus
  • Bleeding from the nose or paranasal sinuses from any cause
  • Swallowed blood from lungs, bronchi, trachea, or larynx (tuberculosis, cancer, pulmonary edema, heavy pneumonia, injury)
  • Large amount of foods containing animal blood

Read more on the causes of black bowel movement.

Invisible (Occult) Blood

May appear in any of above disorders, if bleeding the bleeding is minor. Chronic occult bleeding may result in serious anemia.

Diagnosis of the Cause of Blood in the Stool

When the blood in the stool is suspected, the following has to be done:

1. History

Doctor will likely ask about:

  • Diet to exclude the food as source of the blood
  • Menstruation
  • Eventual blood in the urine
  • Blood: when, how much, how regular, stool color, any mucus, diarrhea, abdominal pain or bloating
  • Diagnoses, investigations and treatments so far
  • Family history of bowel diseases

2. Digital Rectal Examination

Doctor will likely:

  • Look at the anal opening to check for fissures
  • Examine your rectum with a gloved finger to check for palpable mass (polyp, cancer)

Fecal Occult Blood Test (FOBT)

Fecal occult blood test (FOBT) should be done when intestinal bleeding is suspected (after hemorrhoids, anal fissure, menstruation, and urinary bleeding are excluded).

Stool guaiac test (like Hemoccult) may detect daily blood loss of about 10 mL (1). Positive test confirms the presence of the blood in the stool, but not its exact amount or origin. Negative result means, that no blood was in the stool during the testing period, so additional samples at different occasions may be required. Three days before the test, red meat, and foods that contains blood have to be avoided to prevent false positive results;  radishes, horseradish, turnips, cauliflower, uncooked broccoli, figs, melon, citrus fruits, and vit C supplements have to be avoided to prevent false negative results. Test should not be done in bleeding hemorrhoids, urinary bleeding, or during the menstruation. Five samples from each of three consecutive stools have to be taken.

Immunochemical test – iFOBT (like HemSelect) uses antibodies which react with human hemoglobin. The test may detects smaller amount of blood (50 µg/g stool) than guaiac test, but only from the lower GI tract, not from the stomach or above (2). No dietary or drug restrictions are needed before the test; one sample from one or two stools is required. Both guaiac and immunochemical test kits are available in pharmacies without prescription, but testing by a lab or doctor is more reliable.


Expected investigations to find a source (site) of bleeding, when the blood in the stool is RED:

  • Plain X-ray of the abdomen to check for fluid levels, when bowel obstruction is suspected
  • X-ray with  barium enema (lower GI), when ulcerative colitis or Crohn’s disease are suspected
  • Stool culture to check for bacteria, and O&P test for parasites like Giardia, Entamoeba hystolytica
  • Sigmoidoscopy or colonoscopy to check the colon, or capsule (camera pill) endoscopy to check the small intestine

Expected investigations when the blood in stool is BLACK:

  • Esophago-gastro-duodenoscopy (EGD, upper endoscopy) when gastric or duodenal ulcer or cancer, or esophageal varices are suspected
  • Inspection of the nose and throat, CT or MRI of the head and chest – when tumors, heavy infections, or injuries are expected in these areas
  • Blood tests: red blood cells, ferritin, and electrolytes to check for anemia, and electrolytes

Treatment of Blood in Stool

This way:

  • An amount of lost fluid has to be evaluated, and re-hydration (I.V. infusion), or blood transfusion (in massive acute bleeding) started or iron (in chronic bleeding) given.
  • The site of the bleeding has to be found and treated accordingly (small bleeding from the colon can be stopped during a colonoscopy, and esophageal varices during an esophagoscopy)
  • Urgent surgery may be needed in massive bleeding or when bowel obstruction is suspected
About Jan Modric (209 Articles)
Health writer

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