Fecal Occult Blood Test: Everything You Should Know

Medically Reviewed by Jabeen Begum, MD on April 05, 2024
Written by Shawna Seed
7 min read

The fecal occult blood test (FOBT) looks for the presence of blood in the feces, which may be a sign of a problem in your digestive system.

The amount of blood in your stool may be so small that it can be found only by this test, which uses chemicals to detect it. Such blood is considered hidden, or "occult" in medical terms.

The test can find the presence of blood, but it can't tell where the blood came from. If the test is positive, you might have polyps in your colon or rectum. A positive test also could indicate colon cancer. But it can also signal other health conditions, some of which are less serious.

Blood may appear in the stool because of one or more of the following conditions:

Gastrointestinal bleeding may be microscopic (invisible to the eye) or may be easily seen as red blood or black, tarry bowel movements, called melena.

There are different types of fecal occult blood tests, which have different ways of collecting stool samples and testing them.

Guaiac fecal occult blood test

This type, sometimes called gFOBT, uses cards given by your doctor to test samples from multiple bowel movements.

You'll use a clean container to collect multiple stool samples over a few days. Then, you'll use an applicator to spread some of each sample on a card. When they have dried, you'll return the cards to your doctor or a lab either by mail or in person.

Immunochemical fecal occult blood test

The specifics vary by test-kit manufacturer, but usually, you'll use a special tool to collect a sample and place it in the given container. Then, you return the container to your doctor's office or a lab.

This test is more sensitive than the gFOBT. You may not have to collect as many samples for it or limit your diet before taking the test.

Flushable reagent pad or tissue

You can buy this test kit over the counter, without a prescription. It uses a flushable pad or tissue treated with a chemical that detects blood in the feces. You place the pad or tissue into the toilet bowl after a bowel movement. Usually, you'll do this on 3 consecutive days. The color of the pad or tissue will change if it detects blood. Then, you can share the results with your doctor, often on a form that you can mail in.

The FOBT requires the collection of small stool samples. Usually, the samples are a bit of stool collected on the end of an applicator. The samples should be taken 1 day apart because colon cancers may bleed from time to time, rather than consistently.

Fecal occult blood test kit

You can purchase FOBT kits at the pharmacy to perform the test at home, or your doctor may give you the home test during one of your appointments. These tests provide specific instructions, and most offer a toll-free number to call if you have questions.

The stool samples are collected in a clean container and tested for color changes using a test card. Or the samples can be collected in a special container and sent directly to the doctor's office in an envelope for analysis. Your doctor may examine the samples with a microscope or with chemical tests.

The fecal occult blood test results are largely affected by how you prepare for the test, so it is important to follow the instructions carefully.

Certain foods can alter the test results. So, the doctor often recommends a special diet for 48-72 hours before the test. During this period, you should avoid:

  • Raw fruits
  • Raw vegetables
  • Red meat (you can eat chicken and pork)

Also, limit your intake of vitamin C-enriched foods or beverages to less than 250 milligrams per day in the 72 hours leading up to the test.

Your doctor will go over your medicines with you before the test, as you may need to stop taking certain medicines 72 hours before the test.

How often do I need to do the fecal occult blood test?

A colonoscopy is the preferred screening method for colorectal cancer. If you are not having this test done as needed, you should get an FOBT every year, beginning at age 45. This test may be done along with a flexible sigmoidoscopy every 5 years to check for colorectal polyps or cancer.

Because small amounts of blood normally appear in the stool, tests for occult blood are designed to detect larger quantities of blood.

Positive fecal occult blood test

A positive FOBT result means that blood has been found in the stool. Your doctor will have to determine the source of the bleeding, either by doing a colonoscopy or through an examination to find out if the bleeding is coming from the stomach or small intestine.

Negative fecal occult blood test

A negative test result means that no blood was found in the stool sample during the testing period. You should continue to follow your doctor's recommendations for regular screening. If your colon cancer risk isn't elevated, your doctor may want you to repeat the test each year.

The test itself is very safe. There are no reported side effects or complications.

The main issue with the tests is that they are limited as a tool for diagnosing digestive problems. Not all polyps or cancers bleed, so you could get a negative test result and still have a condition that requires treatment. This is called a false-negative result.

 You could also have a positive test caused by something noncancerous, such as a bleeding hemorrhoid, or even swallowing blood from your nose or mouth. This is called a false-positive result. If your FOBT is positive, you'll need further tests. These may include a colonoscopy, which is an invasive procedure that comes with certain risks.

In addition to the foods you eat and medicines you take, some other issues can interfere with the test:

Menstruation. If you take the test during your period, menstrual blood may end up in the sample, leading to a false positive.

Blood in your urine. This condition is called hematuria and can also cause a false positive.

Antiseptics: Using a povidone-iodine antiseptic solution around your rectum also can cause a false-positive result. That's because iodine interacts with the chemical used in some tests to detect blood. Common brand names include:

  • Betadine
  • First Aid
  • GRX Dyne
  • GRx Dyne Scrub
  • Povidex
  • Povidex Peri

Toilet cleaners. If you use a toilet cleaner that contains chlorine, that also could create a false positive. Cleaners that don't generate chlorine can reduce the effectiveness of the chemicals that detect blood, making the test less sensitive.

Yes.

The fecal immunochemical test (FIT) is the test of choice over the fecal occult blood test. This test can be done at home. For this test, you take a sample from your stool with a brush and dab it onto a special card. This test may be easier to do at home than the FOBT. There are no drug or food restrictions.

The stool DNA test or FIT-DNA test is another option for the early detection of colorectal cancer. This test spots cellular changes that could mean you have cancer or precancerous polyps. It also can detect blood in the stool. You use a kit at home to collect a stool sample and send it to a lab for analysis. Colorguard is a brand name for this type of test. This test is nearly as accurate as a colonoscopy for finding colon cancer; both have an effectiveness rate above 90%. But it's much less effective than colonoscopy in finding polyps.

A fecal occult blood test, or FOBT, detects tiny amounts of blood in your feces. It can help find polyps or cancer in your colon. It can also help your doctor diagnose digestive conditions such as colitis, diverticulitis, and Crohn's disease. For the test, you collect samples of your feces using a kit, and then the samples are spread on a card or put into a special container, which you then return to your doctor's office or a lab. Different types of tests use different processes to check for the presence of blood. If the results show blood in your stool — a positive test — then your doctor will recommend other, more extensive tests. The FOBT can only detect the presence of blood. It can't identify the source of the problem.

Is occult blood in stool serious?

A positive result on fecal occult blood test could signal a variety of health conditions, some more serious than others. It can be a sign of colon cancer, but you'll need more tests to find out. It also could mean you have something relatively minor, such as hemorrhoids.

Can constipation cause a positive fecal occult blood test?

Yes. When you're constipated, you may push harder to produce a bowel movement. That can irritate hemorrhoids, causing them to bleed. It can also create small tears in the skin around your rectum — called anal fissures — that can bleed. If your stool is very hard, it might cause tearing and bleeding further up in your anus.