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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:

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

Your doctor will go over your medicines with you before the test, because you may need to stop taking certain medicines 48 hours before the test.

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