The fecal occult blood test (FOBT) is a test used to find blood in the feces, or stool. Blood in the stool may be a sign of colorectal cancer or other problems, such as ulcers or polyps, which are growths that develop on the inner wall of the colon and rectum.
Types of fecal occult blood tests
Currently, there are 2 types of FOBTs: guaiac and immunochemical.
Guaiac-based FOBT. If you have an FOBT in the United States, it is most likely guaiac-based. This test is provided by your doctor’s office or a laboratory and is done at home. 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 blood is in the stool. Then, you send the card back to your doctor’s office or the lab for interpreting. Some guaiac-based FOBTs use flushable pads instead of a card. They are available without a prescription at many drugstores, and results are immediately available to the user.
Immunochemical FOBT. To find if blood is in the stool, the test uses a specialized protein called an antibody. This specific antibody attaches to hemoglobin, the oxygen-carrying part of red blood cells. This test has some benefits over the guaiac test, but it is not widely available in the United States.
Getting ready for a FOBT
How you need to prepare depends on the type of test you take. For the guaiac test, you can’t eat certain foods or take certain medications before performing the test. Substances found in some foods, vitamins, or medicines can cause the test to indicate blood in the stool when there is not. 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 medicines you are currently taking. They might instruct you to make the following changes to your diet several days before the test:
Increasing fiber intake
Avoiding specific foods, such as red meat and certain vegetables
Avoiding certain vitamin supplements, such as vitamin C, and iron
You do not need to make any dietary changes for the immunochemical test. But you may need to avoid certain medicines.
Aspirin or other over-the-counter pain medications and blood-thinning medications may change the test results of both types of FOBTs. Check with your health care team before stopping these medications.
Talk with your health care team about your medical conditions to determine the timing of your FOBT test. For example, the test should not be taken if you have bleeding hemorrhoids, peptic ulcers, or gastritis. And women who are near the time of menstruation should not take the test.
During the procedure
You will need to collect 3 stool samples in a row for the traditional guaiac test. These are stored in a supplied container or placed on a test card with an applicator. You then return the container or card in person or by mail to a laboratory or to your doctor's office.
With the flushable pads, you will drop the pad into the toilet bowl after a bowel movement. You will then repeat this procedure for the next 2 bowel movements. The pads change color when blood is present in the toilet bowl. Write down the results on the provided reply card, and mail it to your doctor’s office or laboratory as instructed.
After the procedure
You can resume your normal activities immediately after the FOBT. After learning the results, talk with your doctor about next steps.
Questions to ask your health care team
Before having an FOBT, consider asking the following questions:
Why do I need this test?
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?
When and how will I learn the results of the FOBT?
What is a false-positive result? What is a false-negative result?
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, what further tests, such as a colonoscopy , will be necessary?
Cancer Screening 
American Association for Clinical Chemistry: Fecal Occult Blood Test and Fecal Immunochemical Test