Fecal Occult Blood Test (FOBT)

A fecal occult blood test (FOBT) is a stool test that looks for hidden (occult) blood in your poop. Healthcare providers use this test to check for bleeding in your gastrointestinal (GI) tract that you might not be able to see in your poop.

The FOBT test kit allows you to safely collect a sample of your own poop at home. You’ll return the sample to a lab. The lab will test it for traces of blood.

Why would I need a fecal occult blood test?

You may take a fecal occult blood test if you have symptoms that suggest hidden GI bleeding. These might include unexplained belly pain or anal pain. Or you may have symptoms of anemia, like fatigue and pallor.

There are a variety of health conditions that might cause hidden bleeding in your GI tract. These can include colon polyps or hemorrhoids. Your healthcare provider may order an FOBT to help confirm or rule out a diagnosis.

You may also take a fecal occult blood test as a form of cancer screening. Healthcare providers recommend regular screenings for colorectal cancer after age 45. (Or earlier, if you have a higher risk based on personal or biological family history.)

It’s important to note that the gold standard for colorectal cancer screening is colonoscopy. Get checked before you have symptoms. Early detection is key. The longer the cancer grows, the harder it may be to treat. Discuss the risks and benefits of each testing method with your provider.

How does a fecal occult blood test work?

There are three types of fecal occult blood test. They look for bleeding in different ways. These are:

• Guaiac fecal occult blood test (gFOBT)
• Fecal immunochemical test (iFOBT or FIT)
• Multitarget stool DNA test with fecal immunochemical testing (MT-sDNA-FIT or FIT-DNA)

Guaiac fecal occult blood test (gFOBT)

The guaiac fecal occult blood test (gFOBT) uses a substance called guaiac (a tree resin) to find blood through a chemical reaction. Guaiac changes color when it interacts with heme. Heme is a part of hemoglobin, which is a component of your red blood cells. Some gFOBT test kits change color when you collect your stool sample at home.

This test finds heme instead of hemoglobin. So, it may spot bleeding higher in your GI tract.

Fecal immunochemical test (iFOBT or FIT)

The fecal immunochemical test (iFOBT) finds blood using antibodies (immunochemicals). These antibodies bind to hemoglobin. The antibodies are attached to a dye or enzyme that produces a color reaction when it binds to its target.

This test is more likely to spot bleeding in your lower GI tract. That makes it a more specific test for colorectal cancer screening.

Multitarget stool DNA test with FIT (MT-sDNA-FIT or FIT-DNA)

This test combines the FIT test (iFOBT test) with a DNA test on the same stool sample. Your poop collects dead cells from the lining of your colon as it passes through. These cells contain your DNA.

Scientists test the DNA for genetic mutations linked to colon cancer, while also checking for hidden bleeding. Your healthcare provider may suggest this test if you have a family history of colorectal cancer.

How should I prepare for a fecal occult blood test?

If you have a FIT test or a FIT-DNA test, you won’t need to prepare beforehand. If you have the gFOBT, you’ll need to avoid certain foods and medications for a few days before taking the test. These include:

• NSAIDs, including aspirin, naproxen and ibuprofen. These can cause temporary bleeding in your stomach, which may contaminate the test results.
• Blood thinners, like heparin, warfarin or clopidogrel. (This depends on your healthcare provider’s advice.) These can make abnormal bleeding more likely and interfere with the test results.
• More than 250 milligrams a day of vitamin C, either from supplements or food sources. These can include plums, cherries, kiwi, rose hip, guava, black currants, bell peppers, strawberries, oranges, broccoli, brussels sprouts, snow peas and chili peppers, among others. Vitamin C can interact with the chemicals in the test and produce a false negative result when it should be positive.
• Red meat, including beef, lamb, veal and pork. Red meat can contain blood components that may trick the gFOBT test and produce a false positive result when it should be negative.

What to expect with a fecal occult blood test

You may have a fecal occult blood test during a visit with your healthcare provider, or you may take one at home. Your provider might prescribe one to you, or you might get one over the counter.

You’ll collect your own stool sample. In most cases, you’ll return the sample to a lab to complete the test. Be sure to follow the specific instructions that come with your test kit. Instructions may differ between kits.

Most FOBT kits include:

• Stool-collecting devices, like long brushes or wooden applicators
• Test cards or tubes to collect your stool sample in
• Waste disposal bags for the collection devices
• An envelope or box to ship your sample

You may need to take one or several samples over a few days. Keep them at room temperature and return them as soon as possible.

The lab will analyze your samples and return your test results to you or your provider. This typically takes about a week.

Your results will be either positive or negative. Your provider will discuss what they mean and what they suggest doing next, if anything.

What does a negative result on a fecal occult blood test mean?

A negative or normal result means that no bleeding was detected. For cancer screening purposes, this means your risk is low. But keep in mind, FOBT isn’t the gold standard method of screening.

Guidelines recommend repeating the test in a year. If you’ve previously had polyps or have a family history of colon or rectal cancer or other genetic conditions, guidelines recommend colonoscopy as the only method of screening.

If you have symptoms that your provider is trying to diagnose, they may suggest trying a different kind of test next.

What does a positive result on a fecal occult blood test mean?

A positive result means blood was detected. This could be from several causes. Bleeding in your colon doesn’t always mean cancer. But it’s something your provider will want to investigate further. Sometimes, the test can produce a false positive result. That’s why it’s important to confirm it.

Bleeding can be a sign of colon polyps. These are growths inside your colon that can turn into cancer. Cancerous and precancerous polyps tend to have fragile blood vessels on the surface that break and bleed easily. Your provider will want to find out if you have colon polyps and to determine what kind they are.

Other conditions that might cause hidden bleeding in your poop include:

• Anal fissures
• Anal STIs
• Diverticulosis or diverticulitis
• Esophageal or gastrointestinal varices
• Hemorrhoids
• Infectious colitis or gastroenteritis
• Inflammatory bowel disease
• Peptic ulcer disease

What does a positive FOBT mean for babies and children?

Children can have most of the same conditions that cause hidden GI bleeding in adults. Children and babies may also have bleeding from causes that don’t affect adults. Some of these include:

• Congenital defects in their intestines, like malrotation or Hirschsprung’s disease
• Food allergies and food protein-induced enterocolitis syndrome
• Necrotizing enterocolitis in preterm infants

What happens after receiving a positive FOBT test result?

If your FOBT is positive, your healthcare provider will want to follow up with an endoscopy. This usually means a colonoscopy to look for colon polyps or other causes of bleeding in your lower intestine.

But you might have an upper GI endoscopy if your symptoms or other tests suggest upper GI bleeding. These procedures allow your provider to confirm, locate and treat the source of your bleeding.