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What You Need to Know about Stool Testing for Colorectal Cancer

Sometimes cancers or polyps bleed. A fecal occult blood test (FOBT) checks for blood in your stool. It can often find polyps and cancer. However, sometimes the test can indicate that there's a polyp or cancer when there isn't. This is called a false positive. Or it can miss a polyp that doesn't bleed. This is called a false negative. 

Another type of FOBT, also called a fecal immunochemical test (FIT), checks for a certain blood component called the globin protein in the stool. This test has fewer false positives, but it still misses polyps that don't bleed.

If you choose one of these tests as your screening test for colorectal cancer, it is important to do it each year. If the test result is positive, you should have a colonoscopy to determine the cause of the bleeding. 

How to do the test

You can do an FOBT at home using an FOBT test kit from your doctor's office or clinic. It's easy to perform, but it can be somewhat unpleasant because it involves collecting samples of your own stool. Each test has slightly different instructions, and it's very important to follow the instructions exactly to get accurate results. Generally, FOBTs work in the following way:

  • You'll start by collecting three consecutive stool samples--sometimes two if you're using an FIT kit. For an FOBT, you can do this by using a clean container or by draping plastic wrap loosely across your toilet bowl to catch your stool. For the FIT you take the samples from stool in the toilet bowl. The process may take several days depending on how often you have a bowel movement.

  • Using the test applicators, you'll take two samples from different parts of each of the stools you collected. Then you'll smear the samples on the testing cards in your kit as directed.

  • Some testing cards come with a chemical solution. When you apply it to the cards that hold the samples, the cards turn blue if there is blood in your stool. If there is no chemical solution, you will have to return the sealed cards to your doctor or lab within two weeks.

  • If you're able to get the results at home and the cards do turn blue, or if you have any questions about the test results, you should contact your health care provider right away.

What to do to get the most accurate results

To make sure you get the most accurate results, follow the instructions in the kit for the basic FOBT. They may advise that you:

  • Don't eat red meat for two to three days before the test. Small amounts of chicken, turkey, or fish are OK.

  • Don't take iron or vitamin C supplements for three days before the test.

  • Don't use products that contain aspirin or use nonsteroidal anti-inflammatory drugs, also called NSAIDs, such as ibuprofen, for seven days before the test.

  • Don't do the test if you are having your menstrual cycle or have active hemorrhoids.

  • Don't test a stool sample that comes in contact with urine or toilet-bowl-cleaning products that turn water blue.

  • If you are using the FIT, there are no drug or dietary restrictions.


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