What is a fecal occult blood test?
A fecal occult blood test (FOBT) looks for small amounts of hidden blood in your stool. These traces of blood aren’t visible to you, but they could signal a problem in your digestive tract. For the test, you’ll take stool samples at home and send them to a lab for testing.
Several digestive conditions cause trace amounts of blood in your stool, including colon polyps or colorectal cancer. FOBT is one option to screen for colon and rectal cancer, but follow-up tests are needed to diagnose the cause if you get a positive result.
Types of fecal occult blood tests
If you need a fecal occult blood test, there are a few different types of tests your doctor might order. Some tests look for trace amounts of blood in the stool, while others look for blood and DNA markers to help detect possible digestive conditions.
No matter the type of test you choose, stool samples are usually collected in the comfort of your own home. If you have a fecal occult blood test as a screening for colon cancer, how often you need the test will depend on the type you choose.
Why is a FOBT performed?
FOBT is most commonly performed as a screening test for colon cancer and rectal cancer. If you don’t have any risk factors for colorectal cancer, including a family history of colon cancer or symptoms, this test may be an option for you starting at age 45. FOBT can also be used when you have signs of a digestive condition.
Your doctor might order a FOBT if you have digestive symptoms or anemia—a condition in which you have a low red blood cell count. This test can help uncover hidden bleeding in the digestive tract so that your doctor can recommend your next steps.
Along with colorectal cancer, some conditions that might cause blood in the stool include:
- Colorectal polyps
- Diverticulosis
- Inflammatory bowel disease, such as ulcerative colitis
- Ulcers
- Hemorrhoids
How often should you have a fecal occult test?
For colorectal cancer screening, the timing of a fecal occult blood test depends on the test you choose—every year or every three years. Guidelines recommend routine colorectal cancer screening for people with an average risk starting at age 45.
If you choose FIT or gFOBT, you’ll need to have the test every year. If you choose a stool DNA test, you should have the test every three years as long as your test is negative. If your test comes back positive, you will have to get a colonoscopy.
Taking a fecal occult blood test
You can get fecal occult blood tests over the counter or through your doctor. But before taking a FOBT, talk with your doctor about if this type of test is right for you. Your doctor can also guide you on any restrictions before the test to help give you the most accurate results.
Each type of test has its own steps, so be sure to follow your kit’s instructions. In general, a FOBT doesn’t require a lot of prep ahead of time and can be done from your home.
Understanding the results
Fecal occult blood tests can only detect the presence of blood in the stool—and, depending on the test, DNA markers. While they are helpful tools for your health and peace of mind, it’s important to understand that these tests can’t diagnose or confirm a condition. Be sure to talk to your doctor about what your results mean for you and your next steps.
- Negative result: A negative result means no signs of hidden blood (or DNA markers if taking a stool DNA test) were found in the stool. For colorectal screening, you’ll follow the guidelines for your next test every year or three years.
- Positive result: A positive result means that blood or DNA markers were detected on your test. This could be caused by many conditions and does not mean you have colorectal cancer. You may have follow-up testing with a colonoscopy to find the cause behind the bleeding or markers.
- False-negative results: Although the performance of these tests is highly dependable, FOBTs are not 100% accurate and can miss polyps, and sometimes signs of cancer, when compared to a colonoscopy.
- False positive results: The test coming back positive doesn’t necessarily mean you have colon cancer or polyps. These tests can send a false signal up to 50% of the time.