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Colorectal Cancer Health Center

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Fecal Occult Blood Test

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When doctors test for fecal occult blood they are testing for the presence of microscopic or invisible blood in the stool, or feces. Fecal occult blood 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 microscopic blood is detected, it is important for your doctor to determine the source of bleeding to properly diagnose and treat the problem.

What Causes Blood to Appear in Stool?

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

  1. Benign (noncancerous) or malignant (cancerous) 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. Abnormalities of the blood vessels in the large intestine
  10. Meckel’s diverticulum, usually seen in children and young adults

Gastrointestinal bleeding may be microscopic (occult blood), 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 three stool samples. 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.

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