Stool Testing for Blood (Fecal Occult Blood Test)

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

What Causes Blood to Appear in Stool?

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.

How Do I Take a Fecal Occult Blood Test?

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.

How Should I Prepare for the Fecal Occult Blood Test?

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.

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How Often Do I Need to Do the Fecal Occult Blood Test?

To allow for the early detection and prevention of colorectal cancer, the fecal occult blood test is recommended yearly for everyone starting at age 50. This test may be done along with a flexible sigmoidoscopy every five years to check for colorectal polyps or cancer.

What Do the Fecal Occult Blood Test Results Mean?

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.

 

WebMD Medical Reference Reviewed by Jennifer Robinson, MD on January 23, 2017

Sources

SOURCE: 

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

National Institutes of Health.
 

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