Fecal Occult Blood Tests

Aprobado por la Junta Editorial de Cancer.Net, 07/2021

The fecal occult blood test (FOBT) is used to find blood in the feces, or stool. An FOBT finds blood in the stool that you cannot see. Blood in the stool may be a sign of colorectal cancer or another medical problem, such as an ulcer or polyps. Polyps are growths that develop on the inner wall of the colon and rectum.

FOBTs are one type of screening tool that doctors use to find colorectal cancer. Regular colorectal cancer screenings are recommended for people age 45 and older. If you have a family history of colorectal cancer or if you have other risk factors of developing colorectal cancer, your doctor may recommend that you start regular screening earlier. Learn more about the colorectal cancer risk factors.

What are the types of fecal occult blood tests?

There are 2 types of FOBTs, both of which you can do at home:

Guaiac-based FOBT. During the test, you place a stool sample on a test card coated with a plant-based substance called guaiac. The card changes color if there is blood in the stool. Then, you send the card back to your doctor's office or the lab for interpreting. Usually, this test is provided to you by your doctor's office or a laboratory.

Some guaiac-based FOBTs use flushable pads instead of a card. They are available without a prescription at many drugstores. Results are available to the user right away.

Fecal immunochemical test (FIT) or immunochemical FOBT. This test uses a specialized protein called an antibody. This specific protein attaches to hemoglobin, the oxygen-carrying part of red blood cells. A sample of the stool is placed in a tube or on a card and sent away to the doctor or laboratory for testing.

The immunochemical test has some benefits over the guaiac test. But both tests are used and can provide information about blood in the stool.

What should I avoid before a guaiac-based FOBT?

Before a guaiac-based FOBT, you cannot eat some foods or take certain medications. Some substances can cause the test to say there is blood in the stool when there is none. This is called a false-positive result.

If you will be taking the guaiac test, talk with your health care team about your diet and the medications you are taking. There are different tests available, and each one has different recommendations for what to avoid.

Most of the time, your health care provider will tell you to increase your fiber intake. You will also most likely need to avoid:

  • Certain vitamin supplements, such as vitamin C and iron

  • Rare red meat, like beef and lamb

  • Non-steroidal anti-inflammatory drugs (NSAIDs), like aspirin, ibuprofen, or naproxen

  • Blood-thinning medications

For some guaiac-based FOBTs, you will need to avoid certain fruits and vegetables. Ask your health care team or refer to the instructions on your FOBT to know what to avoid and for how long.

What should I avoid before an FIT?

Before an FIT, you do not need to make any dietary changes but you do need to avoid certain medications. NSAIDs and blood-thinning medications may change the results of your test. Your health care team will let you know what medications to avoid before taking the test.

When can I take this test?

Talk with your health care team about your medical conditions to decide the best timing for your FOBT. For example, the test should not be taken if you have bleeding hemorrhoids, peptic ulcers, or gastritis. You should also not take the test during your menstrual period. These can give a false-positive result.

How to collect your stool for this test?

Your health care provider will give you instructions for how to best collect the stool samples for your FOBT, whether you are doing a guaiac test or an immunochemical test. There are different brand names for FOBTs, and the instructions can vary for each one.

Guaiac-based FOBT. For a guaiac-based FOBT, you will need to collect 3 stool samples. Usually, these samples need to be collected in a clean container. This means that the sample is not mixed with urine or water from the toilet.

Your test package will include an applicator. Use the applicator to put a sample of your stools on the provided cards or slides. Package the cards as directed and mail them back to your health care provider or the laboratory.

Sometimes, the guaiac FOBT will be done with flushable wipes. After a bowel movement, drop the provided wipe into the toilet. The wipe will change color if blood is present in the stool. Record the results for 3 bowel movements and send the information to your health care provider.

FIT. For an FIT, you will need to collect 2 to 3 stool samples. This stool sample can be collected from the toilet using the applicator tool that was included in your kit. You will apply the sample to the container or card provided. Package the samples as directed and mail them to the laboratory.

What should I expect after the procedure?

You can resume your normal activities right after completing the FOBT. After learning the results, talk with your health care team about whether there are any next steps to take.

Questions to ask your health care team

Before having an FOBT, consider asking the following questions:

  • Why do you recommend this test for me?

  • What are the differences between the guaiac-based FOBT and the immunochemical FOBT? Which test do you recommend and why?

  • How often do I need to take this test?

  • What can I eat or drink before the test?

  • Should I avoid any foods or medications before the test?

  • How accurate is the FOBT in detecting blood in the stool?

  • How accurate is the FOBT in detecting polyps and colorectal cancer?

  • What is a false-positive result? What is a false-negative result?

  • When will my test results be ready? Who will explain the results to me?

  • Do you recommend having another test with the FOBT, such as a flexible sigmoidoscopy exam?

  • If the results indicate blood in the stool, will further tests, such as a colonoscopy, be necessary?

Related Resources

Cancer Screening

Guide to Colorectal Cancer

More Information

U.S. Centers for Disease Control and Prevention: Colorectal Cancer Screening Tests