Fecal Occult Blood Test
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Fecal Occult Blood Test

(FOBT; Stool Occult Blood Test)

En Español (Spanish Version)


A fecal occult blood test (FOBT) is a noninvasive test to detect the presence of hidden blood in the stool (feces). There are two different types of FOBTs:

  • Guaiac smear test—usually available from a laboratory or doctor’s office and interpreted by a medical professional
  • Flushable reagent pads—available over-the-counter in many drugstores

Parts of the Body Involved

The test is used to detect blood leaking into the intestine, usually from the stomach, intestine, or rectum.

Reasons for Procedure

A FOBT is used mainly as part of the colorectal cancer screening process. In some cases, it is used to detect bleeding from an ulcer .

Symptoms of colorectal cancer may include:

  • A change in bowel habits
  • Diarrhea , constipation , or feeling that the bowel does not empty completely
  • Blood (either bright red or very dark) in the stool
  • Stools that are narrower than usual
  • Abdominal discomfort (frequent gas pains, bloating, fullness, and/or cramps)
  • Unexplained weight loss
  • Constant fatigue
  • Vomiting

Colorectal Cancer

Rectal cancer

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The American Cancer Society recommends a yearly FOBT for all men and women, beginning at the age of 50. People should be testing earlier if they have one or more of the following:

Adenomatous Polyp in Colon

Colon polyp

© 2008 Nucleus Medical Art, Inc.

Risk Factors for Complications During the Procedure

A FOBT presents no risks.

What to Expect

Prior to Procedure

False positives can occur with a FOBT if you have minor stomach bleeding from certain medicines, if you have hemorrhoids , or if you eat certain foods that contain blood or chemicals that affect the test. To avoid false positives, do the following:

  • Avoid certain medications and foods:
    • Nonsteroidal anti-inflammatory drugs, such as ibuprofen, naproxen, or aspirin (no more than one adult aspirin per day), for seven days prior to testing; if these are taken daily for medical conditions, consult your doctor before discontinuing
    • Vitamin C exceeding 250 mg per day from either supplements, citrus fruits, or juices for three days before testing
    • Red meats for three days before testing
    • Raw broccoli, cauliflower, parsnips, radishes, turnips, horseradish, and melon for three days before testing
  • Wait until your hemorrhoids are not bleeding.
  • Avoid the test during your menstrual period.
  • Avoid cleaning your toilet bowl for several days before the test since chemicals from the cleanser can affect the test.

During Procedure

The test is completed at home. A single test performed in the doctor’s office following a rectal exam is not adequate.


There is no anesthesia involved in FOBT.

Description of the Procedure

You will set up the kit according to the instructions as you are ready to have a bowel movement. The kit should allow you to collect three samples of stool. Some kits provide a disposable container into which you can pass your bowel movement. Other kits might provide you with tissue paper or plastic wrap that you can lay in the toilet to help keep your stool sample from sinking into the water.

Using thin wooden sticks provided with the kit, you will pick up a very small sample of stool and smear it onto a special card. The card folds over to protect the stool sample. If the test is done in the doctor’s office, your doctor will insert a gloved and lubricated finger into the rectum to check for blood with a chemical test.

After Procedure

You will mail the cards to the clinic or lab in the plastic-lined envelope that comes with the kit. Make sure you have written your name on each card.

How Long Will It Take?

Performing the test should only take a few minutes.

Will It Hurt?

This test does not hurt.

Possible Complications

There are no complications associated with this procedure.

Average Hospital Stay

Hospitalization is not necessary. The test is done at home or during a doctor visit.


Fecal immunohistochemical testing is newer than FOB, performed at home in essentially the same manner. Foods do not alter the test results, and only two specimens may be necessary.


If blood is found in your stool, you may be asked to have additional tests to determine the cause of the bleeding. Although cancer may be one cause of blood in the stool, there are many other causes, including the following:

Call Your Doctor If Any of the Following Occurs

  • Development of any new symptoms
  • Worsening of existing symptoms


American Academy of Family Physicians

National Institutes of Health: SeniorHealth


Canadian Digestive Health Foundation

Health Canada


Can colorectal polyps and cancer be found early? Colorectal cancer screening. American Cancer Society website. Available at: http://www.cancer.org/docroot/home/index.asp . Accessed November 4, 2006.

Causes of occult and obscure gastrointestinal bleeding. American Academy of Family Physicians website. Available at: http://www.aafp.org . Accessed on February 7, 2003.

Diagnostic tests: fecal occult blood test. The Harvard Medical School Family Health Guide website. Available at: www.health.harvard.edu . Accessed on February 7, 2003.

Home stool testing kit a better screening tool. American Cancer Society website. Available at: http://www.cancer.org . Accessed on February 7, 2003.

Pignone M, Campbell M, Carr C, et al. Proposed effects of dietary and medication restrictions during FOBT with guaiac-based tests. Meta-analysis of dietary restriction during fecal occult blood testing. Effective Clinical Practice . 2001;4:150-156.

Last reviewed November 2007 by Daus Mahnke, MD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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