The fecal occult blood test is one of a variety of colorectal cancer screening tests. 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:
- Benign (non-cancerous) or malignant (cancerous) growths or polyps of the colon
- Hemorrhoids (swollen blood vessels near the anus and lower rectum that can rupture, causing bleeding)
- Anal fissures (splits or cracks in the lining of the anal opening)
- Intestinal infections that cause inflammation
- Ulcers
- Ulcerative colitis
- Crohn's disease
- Diverticular disease, caused by outpouchings of the colon wall
- Abnormalities of the blood vessels in the large intestine
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 3 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 a fecal occult blood test?
The test results are largely affected by how you prepare for the test, so it is important to follow the instructions carefully.
Do not perform the test if you have:
- Diarrhea
- Colitis
- Constipation
- Diverticulitis
- Ulcers
- Severe throat irritation
- Hemorrhoid flare-ups
- Your period
Because certain foods can alter the test results, a special diet is often recommended for 48 to 72 hours before the test.
The following foods should not be eaten 48 to 72 hours before taking the test:
- 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, since you may need to stop taking certain medicines 48 hours before the test.
How often do I need to do this 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.
What do the test results mean?
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 (an examination of the entire colon) or by doing an upper endoscopy (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 cancer screening.
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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 5/22/2008...#7143
