What Is a Fecal Occult Blood Test?

Medically Reviewed by Nayana Ambardekar, MD on September 17, 2022
4 min read

A test for fecal occult blood looks for blood in your poop, or feces. It can be a sign of a problem in your digestive system, such as a growth, or polyp, or cancer in the colon or rectum.

If the results show that there is blood (whether you can see it or not), it’s important for your doctor to find the source of bleeding to diagnose and treat the problem.

Blood may show up in your poop because of one or more of the following conditions:

  1. Growths or polyps of the colon
  2. Hemorrhoids (swollen blood vessels near the anus and lower rectum that can rupture, causing bleeding)
  3. Anal fissures (splits or cracks in the lining of the anal opening)
  4. Intestinal infections that cause inflammation
  5. Ulcers
  6. Ulcerative colitis
  7. Crohn's disease
  8. Diverticular disease, caused by outpouchings of the colon wall
  9. Problems in the blood vessels in the large intestine
  10. Meckel’s diverticulum, usually seen in children and young adults

Gastrointestinal bleeding may be microscopic, so you can’t see it. (Doctors call that “occult” blood.) Or you may easily see it as red blood, or black tar-like bowel movements.

There are different kinds of these tests. You can buy some kits at the pharmacy. Or your doctor may give you a home test kit at one of your appointments. They come with instructions. Most list a phone number to call if you have questions.

For some tests, you put a special pad or tissue from the kit into the toilet and tell your doctor if it changes color.

Other tests require you to collect stool samples on more than one day. You then send the samples, in a special container and envelope, directly to your doctor's office for analysis with a microscope or chemicals. You should use newer “high sensitivity” versions of these tests.


You don’t have to “cleanse” your colon like you would before a colonoscopy. But you do need to follow the instructions carefully. Don’t take the test if you have:

  1. Diarrhea
  2. Colitis
  3. Constipation
  4. Diverticulitis
  5. Ulcers
  6. Hemorrhoid flare-ups
  7. Your period

Because certain foods can alter some of the tests’ results, don’t eat these foods for 48 to 72 hours before you take the test:

  • Beets
  • Broccoli
  • Cantaloupe
  • Carrots
  • Cauliflower
  • Cucumbers
  • Grapefruit
  • Horseradish
  • Mushrooms
  • Radishes
  • Red meat (especially meat that is cooked rare)
  • Turnips
  • Vitamin C-enriched foods or beverages

You may need to stop taking certain medicines 48 hours before the test. Ask your doctor about that.

If you have a positive result, that means that it showed blood in the stool. (In this case, “positive” is not necessarily good!)

You’ll then need to get tests to find out where the blood came from. Your doctor may recommend a colonoscopy, and an upper gastrointestinal endoscopy to see if the bleeding is coming from the stomach or small intestine. If these don’t show the source, you may need to swallow a small capsule that takes pictures as it passes through your intestines. It may see areas of bleeding not shown by other tests, especially in the small intestine.

A negative test result means that no blood was found in the stool sample during the testing period. You should continue to follow your doctor's recommendations for regular cancer checks.

You don’t have to take these tests. There are other methods -- such as colonoscopy (in which the doctor uses a flexible tube with a tiny camera on it to check your entire colon); CT colonography (also called a virtual colonoscopy, which is a CT scan of your colon); and tests that check your poop for cancer DNA.

If you decide you want to take fecal occult blood tests, you’ll need to do that every year, and also get other tests by your doctor -- flexible sigmoidoscopy (like a colonoscopy but doesn’t go as far into your digestive system) and barium enema (X-rays of the colon after you get an enema made with barium) -- every five years to check for colorectal polyps or cancer.

Remember, if the test shows signs of blood, you’ll probably still need to get a colonoscopy.