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Fecal Occult Blood Test Information

What Happens During a Fecal Occult Blood Test?

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Updated June 11, 2014

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A fecal occult blood test (FOBT) is a common test that screens for blood in the stool that may not be visible to the naked eye. Occult means "hidden," hence the name "fecal occult blood test." Blood in the stool can signal colon or rectal cancer, polyps, or other digestive problems.

There are two types of occult blood tests that screen for occult blood in the stool: guaiac and the fecal immunochemical test (FIT). The guaiac test utilizes the chemical guaiac to detect occult blood. Immunochemical tests detect blood with the use of antibodies. One advantage of the immunochemical test is that no dietary restrictions or limitations are required, unlike guaiac tests (see "Preparing for the Test" below for more information).

Since the guaiac FOBT is more commonly used in the United States than the immunochemical, information in this article pertains to the guaiac FOBT. (Read About the Fecal Immunochemical Test)

There are no health risks or complications associated with the test. It's non-invasive and involves the voluntary collection of a stool sample at home or at the doctor's office.

A Fecal Occult Blood Test as a Screening Test for Colon Cancer

A FOBT is mainly used as a screening tool to prevent and detect colon cancer. While it can detect occult blood, it is not 100 percent accurate, and false negatives/false positives do occur. However, along with endoscopic screening tests, such as the colonoscopy and sigmoidoscopy, the FOBT is a valuable screening tool for colon cancer.

Preparing for the Test

Before the test, your doctor may want you to avoid certain medications and foods that may potentially cause a false negative or false positive result. The following medications should be avoided before the test, unless your doctor directs otherwise:

NSAIDS: Ibuprofen, naproxen, and aspirin can cause minor stomach or intestinal bleeding that can produce a false positive. If you take these regularly, your doctor will inform you when to discontinue the medication before the test. Your doctor may allow you to take a small dose of aspirin each day, however.

Blood Thinners: Blood thinners such as heparin, warfarin, or aspirin may need to be discontinued before taking a FOBT.

The following foods should not be eaten prior to the test (48-72 hours):

  • Vitamin C from supplements or fresh fruits and juices (no more than 200-250 mg/day)
  • red meat (especially rare red meat)
  • liver
  • lamb
  • cantaloupe and other melons
  • radishes
  • cauliflower
  • uncooked broccoli
  • horseradish
  • cabbage
  • turnips
  • parsnips

Avoid using toilet bowel cleaners a few days before the test as the chemicals may affect the results should the stool sample come in contact with the toilet bowl.

Who Should Not Have a FOBT?

There are certain conditions that may temporarily prevent you from having a fecal occult blood test. Women should avoid the test if they are menstruating, as the menstrual blood can come in contact with stool during a bowel movement, potentially causing a false positive.

If you suffer from hemorrhoids or anal fissures with active bleeding, consult your doctor before taking the test. These too can cause a false positive.

What to Expect During a Fecal Occult Blood Test

In most cases, a fecal occult blood test can be taken in the privacy of your home. Your doctor may send you home with a FOBT kit or may write a prescription for one to pick up at a pharmacy. They are easy to use and all kits come with detailed instructions. If you have any questions, call your doctor.

Testing usually requires sampling from three consecutive bowel movements for the most accurate result. You will collect your stool sample in a dry container or by placing a plastic bag or loose plastic wrap in the toilet (secured under the seat) during a bowel movement. Do not collect stool that has fallen in the toilet water.

The kit contains a small spatula or applicator stick to collect small samples from stools and also cards to smear the samples onto. Using the applicator stick, smear stool onto the card. Place in envelope (also included in kit) and allow to dry. Once dry, return to your doctor or mail them to a lab, as designated by your doctor.

Causes of Blood in Stool

Other than colon or rectal cancer and benign polyps, blood can be present in the stool for a variety of reasons. Hemorrhoids and anal fissures are perhaps the most common cause and usually cause bleeding that can be seen to the naked eye. Other conditions that can cause blood in the stool include:
  • ulcers
  • ulcerative colitis
  • Crohn's disease
  • diverticulosis
  • intestinal infections
  • nosebleeds (swallowing of blood)

If a FOBT is positive, the cause of the bleeding must be determined by your doctor. This may involve having a digital rectal exam (DRE), colonoscopy, sigmoidoscopy, or other medical tests to determine the cause of bleeding.

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