Stool Testing for Blood (Fecal Occult Blood Test)

Medically Reviewed by Minesh Khatri, MD on September 14, 2023
4 min read

The fecal occult blood test (FOBT) looks for the presence of microscopic blood in the feces, which may be a sign of a problem in your digestive system.

Blood may appear in the stool because of one or more of the following conditions:

Gastrointestinal bleeding may be microscopic (invisible to the eye) or may be easily seen as red blood or black tar-like bowel movements, called melena.

The fecal occult blood test requires the collection of 3 small stool samples. Usually the samples are a bit of stool collected on the end of an applicator. The stool samples should be taken one day apart, because colon cancers may bleed from time to time, rather than consistently.

You can purchase fecal occult blood test kits at the pharmacy to perform the test at home, or your doctor may give you the home test during one of your appointments. These tests provide specific instructions, and most offer a toll-free number to call if you have questions.

The stool samples are collected in a clean container and evaluated by detecting color changes on a test card, or by sending the samples, in a special container and envelope, directly to the doctor's office for analysis. Your doctor may examine the samples with a microscope or with chemical tests.

The fecal occult blood test results are largely affected by how you prepare for the test, so it is important to follow the instructions carefully.

Because certain foods can alter the test results, a special diet is often recommended for 48-72 hours before the test. The following foods should be avoided during that time:

  • No raw fruits
  • No raw vegetables
  • No red meat; you can eat chicken and pork 
  • Less than 250 mg per day of vitamin C-enriched foods or beverages in the 72 hours leading up to the test

Your doctor will go over your medicines with you before the test, because you may need to stop taking certain medicines 72 hours before the test.

A colonoscopy is the preferred screening method for colorectal cancer. If you are not having this test done as needed, then you should get a fecal occult blood test every year, beginning at age 45. This test may be done along with a flexible sigmoidoscopy every five years to check for colorectal polyps or cancer.

Because small amounts of blood normally appear in the stool, tests for occult blood are designed to detect larger quantities of blood.

A positive fecal occult blood test means that blood has been found in the stool. Your doctor will have to determine the source of the bleeding, either by doing a colonoscopy or by doing an examination to determine if the bleeding is coming from the stomach or 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 screening.


The fecal immunochemical test (FIT) is the test of choice over the fecal occult blood test. This test can be done at home. For this test, you take a sample from your stool with a brush and dab it onto a special card. This test may be easier to do at home than the FOBT. There are no drug or food restrictions.

 The stool DNA test or FIT-DNA test is another option for the early detection of CRC. This test spots cellular changes that could mean you have cancer or pre-cancerous polyps. It also can detect blood in the stool. You use a kit at home to collect a stool sample and send it to a lab for analysis. This test is still being studied to see how well it works to find colorectal cancer.

Show Sources


Memorial Sloan Kettering Cancer Center: "How to Do a Fecal Occult Blood Test (FOBT)."

National Institutes of Health.

American Cancer Society: "Stool DNA Testing for Colon Cancer."

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