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Colorectal Cancer Screening: Testing for Blood in the Stool

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

Tests, such as the  fecal occult blood test (FOBT) or fecal immunochemical test (FIT), check for blood in your stool. If done every year, they reduce deaths from colorectal cancer. They can often find cancer (and less often polyps). But no test is perfect. These tests can show that there's a polyp or cancer when there isn't. This is called a false positive. They can also miss a cancer or polyp if it doesn't bleed. This is called a false negative. 

If you choose 1 of these tests as your screening test for colorectal cancer, it's important to do it each year. If the test result is positive, you should have a colonoscopy to look for 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 do. But they can be somewhat unpleasant because they involve collecting your own stool. Each test has slightly different instructions. It's very important to follow the instructions exactly to get accurate results.

For the most part, these tests work in this way:

  • Collect the stool sample(s) as instructed. For the FIT, you usually collect 1 stool sample. For the FOBT, you usually collect 3 consecutive stool samples. You do this by using a clean container or by draping plastic wrap loosely across your toilet bowl to catch your stool. The process may take a few days depending on how often you have a bowel movement.

  • Using the test applicators, you usually take 2 samples from different parts of each of the stools you collected. Then you smear the samples on the testing cards in the kit as directed.

  • Close and return the test kit as instructed. Be sure to fill out any personal information.

  • If you have any questions about the test results, contact your healthcare provider right away.

How to get the most accurate results

FOBT. 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 (beef, liver, and lamb) for 3 days before the test.

  • Don't take vitamin C supplements and limit citrus fruits and juice for 3 days before the test.

  • Don't use products that contain aspirin or use nonsteroidal anti-inflammatory drugs, such as ibuprofen, for 7 days before the test.

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

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

Online Medical Reviewer: Jessica Gotwals RN BSN MPH
Online Medical Reviewer: Sabrina Felson MD
Online Medical Reviewer: Todd Gersten MD
Date Last Reviewed: 5/1/2023
© 2023 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.