Colorectal Cancer Screening: Testing for Blood in the Stool

Sometimes colorectal cancers or polyps bleed into the digestive tract, and the blood can enter the stool or feces. Testing for blood in the stool is one way to screen for colorectal cancer.

Tests such as a fecal occult blood test (FOBT) or fecal immunochemical test (FIT) check for blood in your stool. They can often find cancer (and less often polyps). But, sometimes these tests can indicate that there's a polyp or cancer when there isn't. This is called a false positive. Or they can miss a cancer or polyp that doesn't bleed. This is called a false negative. 

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 these tests at home using a test kit from your healthcare provider's office or clinic. They are easy to perform. But they can be somewhat unpleasant because they involve 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, these tests work in the following way:

  • You'll start by collecting 3 consecutive stool samples, or sometimes 2 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 2 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 healthcare provider or lab within 2 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 healthcare provider right away.

How to get the most accurate results

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

  • Don't eat red meat for 2 to 3 days before the test.

  • Don't take vitamin C supplements for 3 days before the test.

  • Don't use products that contain aspirin or use nonsteroidal anti-inflammatory drugs, also called NSAIDs, such as ibuprofen, for 7 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 medicine or dietary restrictions.

If you have any questions about how to do one of these tests, be sure to ask your healthcare provider.

Online Medical Reviewer: Alteri, Richard, M.D.
Online Medical Reviewer: Gersten, Todd, MD
Date Last Reviewed: 6/1/2018
© 2021 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare provider's instructions.