How to diagnose blood in stool

Updated February 21, 2017

Finding blood in your stool can indicate something as simple as hemorrhoids, or as serious as colon cancer. Blood may appear as reddish- or maroon-colored, or as a black tarry stool. It may also be occult blood, or blood that is invisible to the naked eye. It's important to see a doctor and learn the reason for blood in your stool.

Examine your stool. If it is reddish or maroon, it may have recent blood and indicate that you have diverticulitis, an inflammation of small pouches in the lining of your colon. Because it may also indicate colon cancer, you should see your doctor as soon as possible.

Check your stool for a black tarry appearance. Blood from somewhere farther up your digestive tract, like your stomach, duodenum or esophagus, may cause this. It could indicate an ulcer, esophagitis or gastritis. Since these conditions can be serious, you should see your doctor.

Look at your toilet tissue if you've had a large bowel movement that was difficult or painful to pass. If you see spots of red or pink blood it may be simply micro-tears of the tissue around your anus. If it persists, or if there is more blood than mere spots, you should visit your doctor to discuss it.

Consult your doctor if you've been feeling tired, weak or sick for more than a week. He may ask for a stool sample to check for occult blood, only visible under a microscope. There are many possible causes for occult blood, so you may need to undergo further testing if it's present in your stool.

Collect a fresh stool sample if you have either a reddish or maroon stool (hematochezia) or a black tarry stool (melena). Take this to your doctor visit, unless you're sure you can provide a sample at the office.

Make a list of any other symptoms you have, and take it to your doctor appointment.


If you're 50 or older and haven't had a colonoscopy, you should schedule one. It's important to get a baseline look at your colon. Colon cancer can be prevented, and if it's caught early, it can be cured. An ulcer, which may cause a black tarry stool, is treated with H2 blockers, that block the histamines that can cause excess acid in your stomach, and antibiotics to kill the bacteria (H. pylori) that cause most ulcers. False melena, or a black tarry stool not caused by blood, can be caused by taking iron supplements or medications for upset stomach and diarrhea, or even by eating black licorice. Let your doctor know if you consumed any of these in the 48 hours before the black tarry stool appeared. Fecal occult blood may mean you have anemia. Your doctor may need to do some tests to determine the cause of your anemia, if you have it. Iron-deficiency anemia is the most common cause and is easily treatable.

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