What is a Fecal Occult Blood Test?
A fecal occult blood test (FOBT) looks at a sample of your stool (poop) to check for blood. Occult blood means that you can’t see it with the naked eye. And fecal means that it is in your stool.
Blood in your stool means there is bleeding in the digestive tract. The bleeding may be caused by a variety of conditions, including:
- Polyps, abnormal growths on the lining of the colon or rectum
- Hemorrhoids, swollen veins in your anus or rectum
- Diverticulosis, a condition with small pouches in the inside wall of the colon
- Ulcers, sores in the lining of the digestive tract
- Colitis, a type of inflammatory bowel disease
- Colorectal cancer, a type of cancer that starts in the colon or rectum
Colorectal cancer is one of the most common types of cancer in the United States. A fecal occult blood test can screen for colorectal cancer to help find the disease early when treatment may be most effective.
Other names: FOBT, stool occult blood, occult blood test, Hemoccult test, guaiac smear test, gFOBT, immunochemical FOBT, iFOBT; FIT
What is it used for?
A fecal occult blood test is commonly used as a screening test to help find colorectal cancer before you have symptoms. The test also has other uses. It may be done when there is concern about bleeding in the digestive tract from other conditions.
In certain cases, the test is used to help find the cause of anemia. And it can help tell the difference between irritable bowel syndrome (IBS), which usually doesn’t cause bleeding, and inflammatory bowel disease (IBD), which is likely to cause bleeding.
But a fecal occult blood test alone cannot diagnose any condition. If your test results show blood in your stool, you will likely need other tests to diagnose the exact cause.
Why do I need a fecal occult blood test?
Your health care provider may order a fecal occult blood test if you have symptoms of a condition that could involve bleeding in your digestive tract. Or you may have the test to screen for colorectal cancer when you don’t have any symptoms.
Expert medical groups strongly recommend that people get regular screening tests for colorectal cancer. Most medical groups recommend that you start screening tests at age 45 or 50 if you have an average risk of developing colorectal cancer. They recommend regular testing until at least age 75. Talk with your provider about your risk for colorectal cancer and when you should get a screening test.
A fecal occult blood test is one or several types of colorectal screening tests. Other tests include:
- A stool DNA test. This test checks your stool for blood and cells with genetic changes that may be a sign of cancer.
- Colonoscopy or sigmoidoscopy. Both tests use a thin tube with a camera to look inside your colon. A colonoscopy allows your provider to see your entire colon. A sigmoidoscopy shows only the lower part of your colon.
- CT colonography, also called “virtual colonoscopy.” For this test, you usually drink a dye before having a CT scan that uses x-rays to take detailed 3-dimensional pictures of your entire colon and rectum.
There are pros and cons of each type of test. Your provider can help you figure out which test is right for you.
What happens during a fecal occult blood test?
Usually, your provider will give you a kit to collect samples of your stool (poop) at home. The kit will include instructions on how to do the test.
There are two main types of fecal occult blood tests:
- The guaiac fecal occult blood test (gFOBT) uses a chemical (guaiac) to find blood in stool. It usually requires stool samples from two or three separate bowel movements.
- The fecal immunochemical test (iFOBT or FIT) uses antibodies to find blood in stool. Research shows that FIT testing is better at finding colorectal cancers than gFOBT testing. A FIT test requires stool samples from one to three separate bowel movements, depending on the brand of the test.
It’s very important to follow the instructions that come with your test kit. The typical process for gathering a stool sample usually includes these general steps:
- Collecting a bowel movement. Your kit may include a special paper to place over your toilet to catch your bowel movement. Or you may use plastic wrap or a clean, dry container. If you are doing a guaiac test, be careful not to let any urine mix in with your stool.
- Taking a stool sample from the bowel movement. Your kit will include a wooden stick or applicator brush for scraping the stool sample from your bowel movement. Follow the instructions for where to gather the sample from the stool.
- Preparing the stool sample. You will either smear the stool on a special test card or insert the applicator with the stool sample into a tube that came with your kit.
- Labeling and sealing the sample as directed.
- Repeating the test on your next bowel movement as directed if more than one sample is needed.
- Mailing the samples as directed.
Will I need to do anything to prepare for the test?
A fecal immunochemical test (FIT) does not require any preparation, but a guaiac fecal occult blood test (gFOBT) does. Before you have a gFOBT test, your provider may ask you to avoid certain foods and medicines that may affect the results of the test.
For seven days before the test, you may need to avoid:
- Nonsteroidal, anti-inflammatory drugs (NSAIDs), such as ibuprofen, naproxen, and aspirin. If you take aspirin for heart problems, talk with your provider before stopping your medicine. You may be able to take acetaminophen during this time but check with your provider before taking it.
- Vitamin C in amounts over 250 mg a day. This includes vitamin C from supplements, fruit juices, or fruit.
For three days before the test, you may need to avoid:
- Red meat, such as beef, lamb, and pork. Traces of blood from these meats may show up in your stool.
Are there any risks to the test?
There is no known risk to having a fecal occult blood test.
What do the results mean?
If your results from a fecal occult blood test show that you have blood in your stool, it means you likely have bleeding somewhere in your digestive tract. But that doesn’t always mean you have cancer. Other conditions that may cause blood in your stool include ulcers, hemorrhoids, polyps, and benign (not cancer) tumors.
If you have blood in your stool, your provider will likely recommend more tests to figure out the exact location and cause of your bleeding. The most common follow-up test is a colonoscopy. If you have questions about your test results, talk with your provider.