Fecal Occult Blood Test (Guaiac-Based)

Does this test have other names?

gFOBT, stool occult blood

What is this test?

A fecal occult blood test checks a bowel movement (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.

Why do I need this test?

You may need this test because the American Cancer Society recommends that all men and women at average risk for colon cancer start screening tests at age 45. One screening test choice is a fecal occult blood test every year. This simple test can help find colon or rectal cancer. Your healthcare provider may give you the test.

What other tests might I have along with this test?

If the fecal occult blood test shows blood in the stool, your healthcare provider will likely recommend having a colonoscopy to find out the source and nature of the bleeding. 

What do my test results mean?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

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 healthcare provider gives you a kit to help you collect and prepare bowel movement samples for the fecal occult blood test. You often need to collect samples from more than 1 bowel movement–typically 3 in a row. You then mail the samples or take them back to the provider or a lab. Follow the directions your provider gave you and that come with the kit.

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 (from the outside of the stool) on the card or slide you have been given.

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

  4. Flush the unused stool down the toilet.

  5. Repeat this process for the next 2 stools, or as directed. 

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, more commonly can cause a positive test result. Healthcare providers will do more tests to find the cause.

Eating certain food can also affect the test results, even though the fecal occult blood test only detects human blood. Your healthcare 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?

Don't take NSAIDs (nonsteroidal anti-inflammatory drugs), such as ibuprofen or naproxen, or aspirin for 7 days before the test. Acetaminophen is safe to use. If you take aspirin daily to prevent heart disease, talk with your healthcare provider before you stop taking this medicine.

Don't take vitamin C supplements or drink or eat juice or fruits high in vitamin C for 7 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 3 days before testing. It's unlikely, but these foods could change your test result. In some cases, healthcare providers don't put any limits on food because this may make people less likely to do the test. Having the test is the most important thing.

Let your healthcare provider know about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use. 

Online Medical Reviewer: Chad Haldeman-Englert MD
Online Medical Reviewer: Raymond Kent Turley BSN MSN RN
Online Medical Reviewer: Raymond Turley Jr PA-C
Date Last Reviewed: 8/1/2023
© 2000-2024 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.