Fecal Occult Blood Test: What Is It and What Can It Detect?

What is a Fecal Occult Blood Test?

A fecal occult blood test (FOBT) is a type of stool test that is done on a sample of your poop. This test looks for occult (hidden) blood in stool, which can be a sign of bleeding in the digestive tract at any level. 

An FOBT can detect very small amounts of blood in stool that are not visible to the naked eye. If the test is positive for blood, your healthcare provider will follow-up with further testing and treatment. Fecal occult blood tests are commonly done to screen for colorectal cancer. This test is less invasive and less expensive than some of the other colorectal cancer screening tests and it can be done in the privacy of your home.

A fecal occult blood test is also known as a hemoccult test, a stool occult test, an occult blood test, an immunochemical fecal occult blood test or iFOBT, a fecal immunochemical test or FIT, a guaiac smear test, or guaiac-based tests for occult blood (gFOBT).

Types of Fecal Occult Blood Tests

Fecal Immunochemical Test (FIT)

A fecal immunochemical test (FIT) is a type of fecal occult blood test that may be prescribed by a healthcare professional or purchased without a prescription at pharmacies and drugstores. This test must be done every year. 

If someone has colorectal polyps or cancers, the blood vessels in these polyps or cancers are fragile and can be damaged by stool passage, causing bleeding that may not be visible to the eyes. Using this concept, the FIT test checks for blood in the stool from the lower intestines. 

To perform a fecal immunochemical test, you must collect a stool sample after a bowel movement on a stick or applicator which is included in the test kit. You then place the stick in a special container included with the kit and mail it to a laboratory for testing.

Some fecal immunochemical test kits don’t need to be sent to a laboratory and give you the results at home within minutes. In either case, you should discuss the results of the fecal occult blood test with your healthcare provider. 

Guaiac-based Fecal Occult Blood Test (gFOBT)

The guaiac-based fecal occult blood test (gFOBT) requires you to collect three stool samples from bowel movements on three separate days. You need to smear the poop from each bowel movement onto a special card that is included in the test kit. This card is then sent to the laboratory for testing. Similar to the FIT test, gFOBT must be done yearly. 

Unlike the FIT test, the guaiac-based stool test detects stool through a chemical reaction. This means that there are important dietary and medication restrictions you need to follow before the test. For example, you may receive instructions to avoid over-the-counter pain relievers like Advil (ibuprofen) or naproxen (Aleve). Red meats may cause a false positive test result, therefore, your healthcare provider may ask you to avoid it for three days before testing. 

Some guaiac-based fecal occult blood tests are available over-the-counter in pharmacies and drug stores. These tests include flushable pads that are to be placed in the toilet bowl after a bowel movement. The pads change color if blood is present in the stool. You should report the results of your guaiac fecal occult blood testing (gFOBT) to your healthcare provider.

Why Do I Need A Fecal Occult Blood Test?

A fecal occult blood test looks for hidden blood in a sample of stool. Blood in stool can be a sign of colon cancer. Screening tests such as FOBT are one way of looking for colorectal cancers before there are any symptoms. By the time symptoms develop, the cancer has often started to spread. Screening tests can, therefore, help with early detection of cancer when treatment is the most effective. Keep in mind that if the stool test is positive, you may still need a colonoscopy to determine if you have cancer. 

However, fecal occult blood testing is not recommended as a screening method for people who have symptoms of colorectal cancer, such as visible blood in stool, abdominal pain, or changes in bowel habits. FOBT is also not a suitable screening tool for people who are at a high risk of colorectal cancer, for example, due to a history of colon polyps or a family history of colon cancer.

How Often Should You Do Fecal Occult Blood Tests?

The American Cancer Society recommends starting colorectal cancer screening at age 45. According to the National Cancer Institute, a fecal occult blood test (FOBT) is one way of screening for colorectal cancer. Healthcare providers recommend that people who don’t have any symptoms and are at an average risk of colon cancer do stool-based tests such as fecal occult blood tests for colorectal cancer screening once a year. The age of onset may vary if there is a history of digestive or other cancers in your family.

What Is The Most Common Cause of Occult Blood?

Hemorrhoids—also called piles—are the most common cause of occult blood in stool and a positive result on a fecal occult blood test (FOBT). Hemorrhoids are swollen veins in the anus or rectum. These enlarged blood vessels can cause rectal bleeding when you strain during a bowel movement or lift heavy objects. 

What Does A Fecal Occult Blood Test For?

A fecal occult blood test (FOBT) can detect health conditions that cause gastrointestinal bleeding, for example: 

• Hemorrhoids 

• Diverticulosis

• Inflammatory bowel disease (IBD) (ulcerative colitis)

• Colorectal cancer

• Colorectal polyps (small growths that are not cancer, but some precancerous polyps can become cancer)

• Esophagitis

• Gastritis

• Peptic ulcer disease (bleeding stomach ulcer)

• Stomach cancer

Overview of What To Expect

Do I Need To Prepare For The Test?

Some, but not all, fecal occult blood tests require you to prepare before the test. This is because certain foods, medications, and supplements can cause false positive or false negative test results, especially with the guaiac-based fecal occult blood test (gFOBT). Talk to your healthcare professional and read the test kit instructions carefully to find out if you need any special preparations. 

Even if you have done these at-home stool tests before, make sure you follow the instructions that come with the kit since different kits may be different instructions. Talk to your healthcare provider if you still have questions on what to do. 

What Should I Avoid Before A Stool Test?

Depending on the type of fecal occult blood test, you may need to avoid certain medications and foods for 48-72 hours before the test, including:

• Certain raw vegetables like radishes, beets, turnips, broccoli, carrots, and cauliflower.

• Certain raw fruits such as grapefruit, melon, and cantaloupe.

• Red meat, especially rare red meat.

• Certain dietary supplements, such as vitamin C and iron.

• Pain medications, such as aspirin and other nonsteroidal anti-inflammatory drugs like ibuprofen (Advil, Motrin) and naproxen (Aleve).

Also, avoid scheduling any dental work for three days before your fecal occult blood test. Dental procedures that cause significant bleeding can cause you to swallow some blood, and this blood can show up in your stool and cause a false alarm.

What Happens During A Fecal Occult Blood Test?

Your doctor may give you a fecal occult blood test kit or arrange to have it sent to you by mail. Each type of fecal occult blood test requires you to collect samples in a specific way. Follow the specific instructions that come with the test kit. Everything you need is typically included in the kit, such as the device to collect the stool sample and the card or container to send the stool samples to the laboratory. The kits also include instructions on how best to take a stool sample from the toilet bowl. 

Understanding Test Results

What Do Fecal Occult Blood Test Results Mean?

The result of your fecal occult blood test can be one of the following:

• Negative Result: This means no blood was found in your stool. If you don’t have any symptoms and are at an average risk of developing colorectal cancer, your doctor may recommend repeating the test annually.

• Positive Result: This means blood was found in your stool. A fecal occult blood test can detect blood in stool, but it does not indicate the source of the bleeding. 

What Happens If Stool Occult Blood Is Positive?

If you have a positive test result on FOBT, your healthcare professional will recommend more testing, typically a colonoscopy, to find the source and location of the gastrointestinal bleeding. Remember, a positive FOBT result does not necessarily mean you have colorectal cancer. It only means you have blood in your stool and the cause needs to be investigated. 

Are There Any Risks To The Test?

A fecal occult blood test is not 100% sensitive. The test sometimes says there is blood in stool when there isn’t—this is called a false positive result. False positive test results can lead to unnecessary concern and more tests. As mentioned above, certain foods, medications, and vitamin supplements can lead to a false positive result on FOBT. In addition, false positive test results can occur from contamination, for example, with povidone-iodine antiseptic solution (Betadine) used in certain medical procedures. 

A fecal occult blood test can also miss blood that's actually present—this is called a false negative result. It can result in a missed cancer diagnosis and timely treatment.

Other Tests For Colorectal Cancer Screening

In addition to a fecal occult blood test, other screening tests are available that can also detect colorectal cancer and bleeding in the gastrointestinal tract, such as:

Imaging Tests

• Sigmoidoscopy — a minimally invasive diagnostic test that allows your doctor to view the inside of your sigmoid colon (the lowermost part of your colon or large intestine) and rectum

• Colonoscopy — a diagnostic procedure that allows your doctor to visualize your entire colon, rectum, and anus.

• Virtual colonoscopy (CT colonoscopy or colonography) — an imaging test that uses X-rays to create images of your colon. It is a type of CT scan and is less invasive than a conventional colonoscopy.

Stool Tests

• Stool DNA test (Cologuard) — an at-home screening test that looks for tiny amounts of blood in stool and also checks for DNA changes found in precancerous and cancerous cells. Unlike the FIT and the guaiac-based tests, which must be done yearly, stool DNA tests such as Cologuard can be done every three years in the privacy of your home. 

Blood Tests

There are two FDA-approved blood tests for colorectal cancer screening (Shield and Epi proColon). These tests are recommended for people at an average risk of colon cancer. They are done on a sample of blood in a laboratory and check for certain DNA changes that are seen in pre-cancer or cancer cells.

Advantages of Fecal Occult Blood Testing

Many people prefer stool tests for occult blood, such as fecal immunochemical testing (FIT) and guaiac-based fecal occult blood test (gFOBT), because:

• You don’t need to go to a doctor’s office. This simple test can be done at home.

• Dietary restrictions are not always required before the stool test, such as the FIT test.

• Some tests require you to collect only one stool sample. 

• Results may be available within minutes.

• Stool tests have good sensitivity in detecting occult blood.

• They are less invasive than a sigmoidoscopy or colonoscopy.

Your healthcare professional will recommend fecal occult blood tests (immunochemical tests or guaiac-based tests) or some other colorectal cancer screening test, depending on your personal health history and family history.