What Is a Fecal Occult Blood Test?

Testing a sample of your stool with a fecal occult blood test can provide an early warning of colorectal cancer. Here's what you should know.

Talking about your poop, much less collecting it in a box and putting it in the mail, might not be your idea of fun. But there are good reasons for having a fecal occult blood test (FOBT), including catching colorectal cancer early.

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What is a fecal occult blood test?

"Fecal" refers to feces, otherwise known as stool or poop. "Occult" means hidden. A fecal occult blood test finds miniscule amounts of blood in your stool that are too small for you to see.

FOBT is a screening test for colorectal (colon) cancer. Sometimes doctors use it to find the cause of bleeding in your digestive tract from other conditions, such as inflammatory bowel disease (IBD).

There are two types of fecal occult blood tests:

• The guaiac fecal occult blood test (gFOBT) uses a card coated with a plant-based substance to detect blood in your stool.
• The fecal immunochemical test (FIT) is a newer test that uses a special protein called an antibody to detect blood in stool.

FOBT isn't the same as the Cologuard stool test you might have seen advertised on TV. FOBT detects only blood, while Cologuard looks for cancer DNA in stool.

When might I need FOBT?

FOBT is one of the tests that experts recommend you get regularly to screen for colorectal cancer. Other screening options are:

• Colonoscopy
• Flexible sigmoidoscopy
• Virtual colonoscopy

Having a stool-based test may be an option if you'd rather not undergo a procedure like colonoscopy. If you're going to screen with only a FOBT, most medical groups recommend that you have this test every year, starting at age 45. You may want to start testing earlier if you're at higher risk because of Crohn's disease, ulcerative colitis, or a family history of colorectal cancer.

Your doctor might also recommend a FOBT if you have possible bleeding in your GI tract from another condition, such as IBD.

How do I prepare for a fecal occult blood test?

Some foods and medications can cause a false-positive result, which means the test finds blood when there is none. In the days before your test, your doctor might suggest that you avoid:

• Rare red meats such as beef, pork, and lamb
• Vitamin C and iron supplements
• Nonsteroidal anti-inflammatory drugs (or NSAIDs) such as ibuprofen (Advil, Motrin) and aspirin
• Blood-thinning medications

Wait to have this test if you're bleeding from an ulcer or hemorrhoids. Women should avoid having a FOBT during their period because menstrual blood can make the results harder to interpret.

How is a fecal occult blood test done?

You will need to collect a stool sample at home. You can buy a kit yourself without a prescription or get one from your doctor. The kit will include instructions on how to collect the sample, plus all the tools you'll need to gather and prepare it.

Depending on which of the two tests you take, you'll need to collect either two or three stool samples. Then you'll put the samples on a card or place them into a tube or other container. Package them up as directed and mail them to either your doctor or a lab.

What do my results mean?

No blood in your stool is a negative test result.

Blood in your stool is an indication that you have bleeding somewhere in your GI tract, although it doesn't necessarily mean that you have cancer. Hemorrhoids, ulcers, and a few other conditions also cause this symptom. Your doctor will probably recommend having more tests to find the cause of the bleeding.

How accurate is a fecal occult blood test?

Screening test accuracy is measured in sensitivity — the likelihood of the test accurately detecting a positive result and specificity — the likelihood of it accurately detecting a negative result. Research finds that a FOBT has a sensitivity of 79 percent and a specificity of 94 percent.

FOBT is accurate at detecting colorectal cancer but not nearly as accurate as a colonoscopy. Colonoscopy not only identifies about 95 percent of these cancers but also enables your doctor to remove any precancerous or cancerous polyps that are found.

Numbers aside, the best test is the one you're willing to have. If you're more likely to use a home stool test than undergo a colonoscopy procedure, choose a FOBT.

What you can do

Talk to your primary care doctor or gastroenterologist about your colorectal cancer screening options. If your doctor recommends a FOBT, ask why they're suggesting it. After you have an FOBT, make sure you understand both your results and next steps.

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