The fecal occult blood test (FOBT) is a diagnostic examination used to detect blood in the feces (stool). Blood in the stool may be a sign of colorectal cancer or other problems, such as polyps (growths that develop on the inner wall of the colon and rectum) or ulcers.
Currently, there are two types of FOBTs—guaiac and immunochemical. The majority of FOBTs in the United States are guaiac-based. A guaiac FOBT uses a card coated with guaiac, a plant-based substance that is used as an indicator of the test results. After the stool sample has been placed on the test card, the test is processed, and the card changes color if blood is present.
An immunochemical FOBT uses a slightly different process to detect blood in the feces. This test uses an antibody (a specialized protein) that attaches to hemoglobin (the oxygen-carrying part of the red blood cell), indicating the presence of blood in the stool. This test has some benefits over the guaiac test, but it is not widely available in the United States.
A guaiac FOBT is done in the privacy of your home with either a traditional test or flushable reagent pads, which are placed in the toilet after a bowel movement, eliminating the need to handle the stool. The traditional test is interpreted by a medical professional, and these tests are typically available from your doctor or a laboratory. The flushable pads are available without a prescription at many drugstores, and results are immediately available to the user.
Questions to ask your doctor
Before having a FOBT, consider asking your doctor the following questions:
- Why do I need this test?
- What are the differences between the guaiac FOBT and the immunochemical FOBT? Which test do you recommend and why?
- What may 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 (such as a flexible sigmoidoscopy exam) performed in conjunction with the FOBT?
- What further tests (such as a colonoscopy) will be necessary if the results indicate blood in the stool?
Preparing for the procedure
The preparation method depends on the type of test you take. The guaiac test requires that you not eat certain foods or take certain medications before performing the test because substances found in some foods, vitamins, or medications can cause the test to give a positive result (indicating the presence of blood in the stool) even when no cancer is present (this is called a false-positive result).
If you are taking the guaiac test, talk with your doctor about your diet and the medications you are currently taking. Your doctor might instruct you to change your diet several days before the test, increasing fiber intake, while avoiding certain foods and vitamin supplements, such as red meat, certain vegetables, vitamin C, and iron. The immunochemical test requires no dietary restrictions before taking the test.
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 doctor before stopping these medications.
Talk with your doctor 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. In addition, women who are near the time of menstruation should not take the test.
During the procedure
The traditional guaiac test requires you to collect three consecutive stool samples. 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 two 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.
After the procedure
You can resume your normal activities immediately after the FOBT. After learning the results, talk with your doctor about next steps.