A fecal occult blood test (FOBT) checks for small amounts of blood in your stool that cannot be seen with the naked eye. If blood appears on your test, it likely means that you’re bleeding somewhere in your gastrointestinal tract.

Additional tests, like a colonoscopy, are usually needed to find the cause and source of bleeding. Healthcare professionals use FOBTs as a regular part of colon cancer screening as well as to help diagnose other gastrointestinal conditions.

Keep reading to learn more about when this test is used and why it’s important.

Healthcare professionals use FOBTs to find bleeding in your digestive tract. Bleeding can be a sign of colorectal cancer or other gastrointestinal disorders.

Colorectal cancer is the third most common cancer in the United States and the second most common cause of cancer death. The American Cancer Society recommends that all people over age 45 get screened for colorectal cancer.

Two types of FOBTs are widely available:

  • Guaiac-based FOBTs (gFOBTs). You place your stool sample on a card coated with a substance called guaiac. This substance changes color if it contacts blood.
  • Fecal immunochemical test (FIT). You place your stool sample on a card or in a container coated with a protein that attaches to the oxygen-carrying part of red blood cells (hemoglobin).

The United States Preventive Services Task Force recommends FIT or gFOBT testing yearly to screen for colon cancer. If irregular results are found, a colonoscopy is usually required to confirm the results by finding the location and cause of any bleeding that exists.

FOBTs can also be used to identify active bleeding from other gastrointestinal conditions that cause bleeding. These include:

Your doctor will provide you with specific instructions for your test. It’s important to follow these instructions to get accurate results.

Taking certain supplements or medications can cause a false positive when taken up to 7 days before testing. A false positive means the test suggests you have blood in your stool when it’s not there.

Do not stop taking any of your medications without your doctor’s approval first. Make sure to discuss with your doctor if you’re taking any of these medications:

It’s important to tell your doctor if you have blood in your urine, have hemorrhoids that are bleeding, or are menstruating before you collect your stool sample. Your doctor will likely recommend waiting until these sources of bleeding stop to avoid a false positive.

Doctors often recommend increasing your fiber intake in the days leading to your test to stimulate bowel movements.

What foods to avoid for an occult blood test

Consuming certain foods within 3 days can cause a false positive on a gFOBT. These foods include:

  • broccoli
  • cantaloupe
  • cauliflower
  • horseradish
  • parsnips
  • rare or red meat
  • red radishes

No dietary restrictions are required for FIT, according to a 2020 review.

The method of collecting your sample depends on the type of test you’re receiving and the manufacturer’s instructions. Here’s an idea of what to expect.


For a gFOBT, you’ll be given a test card that you can use for your sample. You’ll collect stool samples from two or three bowel movements and put them into a clean container. Your samples cannot mix with urine or water from your toilet.

You’ll use a special applicator to smear the stool on a specified area on the card. Once your sample is dry, you’ll return it to your doctor or a laboratory.

Some tests use flushable pads instead of cards. These tests provide your results immediately.


For a FIT, you’ll collect a sample of your stool from your toilet using a special spoon or another device that comes with the kit. You’ll store your sample in the special collection container that came with your kit before returning it to your doctor or laboratory.

A positive test indicates that blood was found in your stool, but a positive result by itself cannot diagnose colorectal cancer or another gastrointestinal problem.

When a positive FOBT result is found, a colonoscopy is usually needed to find the source of the bleeding. Your doctor may also recommend an upper gastrointestinal endoscopy to check your small intestines and stomach.

Your doctor can help answer any questions you may have about your results. Questions you may want to ask include:

  • What further tests will I need?
  • What are the chances of a false positive?
  • What might be causing the bleeding?
  • How long until I can undergo further testing?

A negative FOBT result means that there wasn’t any blood detected in your sample. Not all colorectal cancers bleed. Even if you have a negative result, it’s still possible to have cancer.

Your doctor can best advise you about whether you need any further testing. While talking with your doctor, you may want to ask:

  • What are the chances of a false negative?
  • Should I undergo further testing?
  • Should I undergo any other tests?

Living with troubling test results

Receiving discouraging results can be difficult, but there are many resources available to you to make coping easier. Here are some that you may find useful:

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FOBT has been found to be a reliable and effective screening test for colorectal cancer. A large 2021 study from Japan found that FOBT and colonoscopy screening reduces the risk of developing or dying from colorectal cancer.

In a 2019 review of studies, researchers found that FITs were generally better at detecting colorectal cancer than guaiac-FOBTs. They found FITs correctly identified:

  • colorectal cancer in the proximal colon 67 percent of the time
  • colorectal cancer in the distant colon or rectum 72 percent of the time
  • the absence of colorectal cancer 95 percent of the time
  • the absence of advanced adenoma 93 percent of the time
  • advanced precancerous polyps 94 percent of the time

A 2020 review of studies found that FOBT is poor for diagnosing iron deficiency anemia, with 42 percent of people receiving false negatives.

Fecal occult blood tests (FOBTs) check for small amounts of blood in your stool that can suggest a problem with your gastrointestinal tract. They’re used as a part of regular cancer screening and can help diagnose other conditions that cause bleeding, like ulcerative colitis or Chron’s disease.

Your doctor can help you interpret the results of your test and best advise you about whether further testing is needed.