Patient Basics: Fecal Occult Blood Test (FOBT and FIT)

A fecal occult blood test checks a small sample of stool for blood you can’t see. “Occult” means hidden, and occult blood in stool can come from polyps or cancer in the colon, so this test is one of the main ways to screen for colorectal cancer. You collect the sample at home and mail it or drop it off. If the result is positive, you need a colonoscopy to find out why.

Key points

• Most adults should start colorectal cancer screening at age 45. A yearly stool test is one of the accepted choices.
• The most common version today is the FIT (fecal immunochemical test). It needs one sample and no changes to what you eat.
• The older guaiac test needs samples from three bowel movements and some diet and medicine changes beforehand.
• A positive result doesn’t mean you have cancer, but it does mean you need a colonoscopy soon.
• If you have symptoms such as rectal bleeding, or a high risk of colon cancer, a stool test isn’t the right test. Talk to your doctor.

Why a fecal occult blood test is done

Colorectal cancer often grows from small growths called polyps over many years. Polyps and early cancers sometimes bleed tiny amounts into the stool. Finding that blood early leads to a colonoscopy, where polyps can be removed before they turn into cancer, and cancers can be found when they are easier to treat.

US guidelines advise screening for people at average risk from age 45 to 75. After 75, you and your doctor decide based on your health and past results. A stool test is a good choice if you want to avoid the bowel prep and sedation of a colonoscopy. It only works if you repeat it every year.

Types of stool tests

• FIT. This test uses antibodies to find human blood. It is mainly sensitive to bleeding from the lower bowel. You need one sample, and food and medicines don’t affect it.
• Guaiac test (gFOBT). Also called a stool guaiac test or Hemoccult test, this older test uses a chemical reaction. Some foods and medicines can make it falsely positive or negative, so you need to prepare. It takes samples from three separate bowel movements.
• Stool DNA test with FIT (such as Cologuard). This test looks for blood and for changes in DNA shed by polyps or cancer. It is usually repeated every 3 years. It finds more cancers than FIT alone but has more false alarms.

Before the test

For a FIT or stool DNA test, you don’t need to change your diet or medicines.

For a guaiac test, follow the instructions in your kit. They often ask you to do the following for a few days before and during sample collection:

• Avoid red meat, such as beef and lamb.
• Avoid vitamin C supplements and large amounts of citrus juice.
• Avoid aspirin, ibuprofen and naproxen if you can. Keep taking aspirin your doctor prescribed for your heart, and ask before stopping any medicine.

For any stool test, wait to collect a sample if you have your period, bleeding hemorrhoids or blood in your urine. These can cause a false positive.

How a fecal occult blood test is done

You do the test at home with a kit from your doctor’s office or lab.

1. Read the kit instructions first and write your name and the date where asked.
2. Pee before you collect, so urine doesn’t mix with the stool.
3. Catch the stool before it touches the toilet water. Most kits include a paper sheet that sits across the toilet seat. A clean, dry container also works.
4. For a FIT, scrape the tip of the probe across the stool, then push it back into its tube and close it tightly. For a guaiac test, smear a thin layer onto each window of the card with the stick provided.
5. Flush the rest of the stool and wash your hands.
6. Mail or drop off the sample within the time listed on the kit. Samples left too long can give a wrong result.

A stool DNA test asks you to collect a whole bowel movement in a container that clips onto the toilet, then ship the full kit.

Possible risks

The test itself has no physical risk. Its limits are a missed cancer or polyp (a false negative) and a positive result that turns out to have a harmless cause (a false positive). That’s why the test must be repeated on schedule.

Your results

Results usually come back within 1 to 2 weeks.

• Negative. No blood was found. Repeat the FIT or guaiac test in one year, or a stool DNA test in 3 years.
• Positive. Blood was found. Most people with a positive result don’t have cancer. The blood may come from polyps, hemorrhoids, small tears near the anus, inflammation, or stomach ulcers (with the guaiac test). You need a colonoscopy to find the cause. Don’t repeat the stool test in place of a colonoscopy. Try to have the colonoscopy within a few months.

When to call your doctor

• You see blood in or on your stool, or have black, tarry stools
• You have a change in bowel habits that lasts more than a few weeks
• You have unexplained weight loss, belly pain or tiredness
• You have a parent, brother, sister or child who had colorectal cancer or advanced polyps
• You haven’t been told your result after 2 weeks

Common questions about the fecal occult blood test

What does occult blood in stool mean?

It means there is blood in your stool in an amount too small to see. It can come from anywhere in the digestive tract, including hemorrhoids, polyps, ulcers, inflammation or cancer. A positive test needs a colonoscopy to find where the blood is coming from.

How accurate is a fecal occult blood test?

A single FIT finds about 3 out of 4 colorectal cancers but misses many polyps. Polyps and cancers bleed on and off, so one test can miss them. Repeating the test every year gives it more chances to catch a problem that didn’t bleed the first time.

Is a stool test as good as a colonoscopy?

A colonoscopy finds more polyps and lets the doctor remove them during the same exam. A yearly FIT is still a good screening choice for people at average risk who would rather skip the bowel prep. Any screening test you complete beats putting off screening.

Questions to ask your doctor

• Is a stool test a good screening choice for me, or should I have a colonoscopy?
• Which type of stool test are you giving me, and do I need to prepare?
• How and when will I get my results?
• If my result is positive, how soon can I get a colonoscopy?
• When should I test again?

Learn more

• Fecal occult blood test (FOBT) (MedlinePlus, U.S. National Library of Medicine)
• Screening for colorectal cancer (Centers for Disease Control and Prevention)

This guide is general information and doesn’t replace advice from your own doctor. Last updated October 2026.