Health Encyclopedia


Tests & Procedures

Fecal Occult Blood TestAnálisis de sangre oculta en las heces

Fecal Occult Blood Test

Does this test have other names?

FOBT, stool occult blood

What is this test?

A fecal occult blood test checks a stool sample for blood that can't be seen with the naked eye. Blood in the stool is a sign of bleeding in the digestive tract. This could indicate cancer, polyps, hemorrhoids, diverticulosis, or inflammatory bowel disease, also called colitis. 

Why do I need this test?

The American Cancer Society recommends that all men and women at average risk for colon cancer start screening tests at age 50. One screening test option is a fecal occult blood test every year. This simple test can help find colon or rectal cancer.

What other tests might I have along with this test?

If the fecal occult blood test shows blood in the stool, you will need a colonoscopy to find out the source and nature of the bleeding. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

A fecal occult blood test uses chemicals to find out if there is blood in a sample of feces. If your test is negative, your result is normal.

If your test is positive, you had blood from your digestive tract in your stool sample. Additional testing, such as a colonoscopy, can help find out the location, cause, and extent of the bleeding. 

How is this test done?

Your health care provider gives you a kit to help you collect and prepare bowel movement samples for the fecal occult blood test. The test requires collecting samples from more than one bowel movement – typically, three in a row. You then mail the samples to a lab or returning them to your doctor's office.

Collecting and preparing the samples typically follows these steps:

  1. Collect one of your stools in a dry container. Don't allow urine to mix with it.

  2. Use a wooden applicator to put a small smear of stool on a card or slide.

  3. Flush the unused stool down the toilet.

  4. Seal the sample and write your name and date on it.

  5. Repeat this process for the next two stools, or as instructed. 

Does this test pose any risks?

This test poses no known risks.

What might affect my test results?

A positive result on a fecal occult blood test doesn't mean you have cancer. Other health conditions, such as ulcers or hemorrhoids, can cause a positive test result. Doctors will do more tests determine the cause.

Eating certain foods can affect the test results, even though the fecal occult blood test only detects human blood. Your health care provider may ask you to avoid certain foods a few days before the test to lower the chances of getting a false-positive result. 

How do I get ready for this test?

Do not take nonsteroidal anti-inflammatory drugs, such as ibuprofen (Advil) or naproxen (Aleve), or aspirin for seven days before the test. Acetaminophen (Tylenol) is safe to use. If you take aspirin daily to prevent heart disease, talk with your health care provider before you stop taking this medication.

Do not take vitamin C supplements or drink or eat juice or fruits high in vitamin C for seven days before the test. Vitamin C can cause a false-negative test result.

Don't eat red meat, such as beef, lamb, pork, and liver, for three days before testing. Although it's unlikely, these foods could alter your test result. In some cases, health care providers don't give any restrictions because this may make patients less likely to do the test. Having the test is the most important thing.


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