Occult blood refers to small traces of blood that a person cannot see in their stool. The presence of blood in the stool could be a sign of an underlying condition. Health experts often use this test as a screening tool for the early detection of bowel cancer.

Stool samples can provide valuable information about a person’s gastrointestinal system. The presence of blood in the stool is a sign that indicates bleeding somewhere in the digestive tract. This can happen due to a variety of conditions, ranging from mild irritation to serious conditions such as colorectal cancer, also known as bowel cancer.

This article discusses occult blood stool tests, including their purpose, how people can prepare for it, and how to perform the test.

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Fecal occult blood tests (FOBTs) describe a noninvasive lab test that checks stool samples for the presence of blood. Occult blood refers to microscopic blood that the naked eye cannot detect. Different types of fecal occult blood tests are available, which vary in their method of detecting blood.

Guaiac fecal occult blood test (gFOBT)

This test uses a chemical called alpha guaiaconic acid or guaiac to detect blood in the stool. The testing card contains guaiac. A chemical reaction occurs when blood is present in the sample, and the paper turns blue, indicating a positive result. It may be advisable for a person to perform a gFOBT test once a year after they are 45 years old or earlier if they have other risk factors for colon cancer.

Fecal immunochemical test (FIT)

This test uses antibodies that bind to hemoglobin in the stool sample. Hemoglobin is a protein in red blood cells that gives blood its red color. It is also advisable to perform this test once a year after the age of 45 or earlier if a person has other risk factors for colon cancer.

Stool DNA test (FIT-DNA test)

This test uses a combination of antibodies used in the FIT test and altered DNA material associated with colorectal cancer. Doctors recommend this test every 3 years after the age of 45 or earlier for people with other risk factors for colon cancer.

Health experts often use fecal occult blood tests to screen for cancer before a person shows any symptoms. The American Cancer Society recommends people with an average risk of colorectal cancer undergo screening at age 45.

However, doctors may also order this test when there is a concern for gastrointestinal bleeding caused by other conditions, such as:

FOBTs may also help find the cause of anemia. Doctors can also use these tests to identify if a person has inflammatory bowel disease (IBD) rather than irritable bowel syndrome (IBS), a similar condition that does not cause bleeding and anemia.

However, a FOBT alone cannot diagnose any condition. If a person receives an irregular result, a doctor may request other tests to determine the potential cause, such as an imaging test.

Some foods and drugs may affect the results of the test. The doctor will provide specific instructions to ensure the accuracy of the test.

If possible, it is preferable for a person to avoid certain medications for up to 7 days before testing to prevent false-positive results. These can include:

It is also advisable for a person to avoid the following foods 3 days before testing to prevent false-positive results:

False negatives can also occur in people taking more than 250 milligrams of vitamin C daily.

A person with certain conditions may consider discussing them with their healthcare professional and rearranging when to collect a sample to prevent false-positive results. This can include:

It is also essential for a person to inform the healthcare professional of any vitamins, supplements, herbs, and prescription and nonprescription drugs that they are taking.

How a person will collect and handle the stool sample depends on the type of test and the manufacturer’s instructions.


To perform a gFOBT, a person should collect and store their stool sample in a dry, clean container. It is important not to let urine or water mix with the stool to avoid contamination. They need to use the applicator that comes with the kit to smear a small amount of the stool on a specified area inside the testing card or place it into a tube that comes with the kit.

Depending on the instructions, a person may need to reuse the applicator brush to get a second sample from a different part of the stool to repeat the test on their next bowel movement.

The test may also require samples from 2–3 separate bowel movements. A person should then label and seal the samples as directed and transport or mail them to the laboratory for testing.


To perform a FIT, a person must first flush the toilet 2–3 times before collecting the sample. A person will then have their bowel movement as usual and should not flush the toilet.

Using a brush from the kit, a person can scrape the stool’s surface for a few seconds. After gently shaking the brush to remove excess water or clumps of stool, they can then rub the brush on the indicated space.

Some kits may require the person to apply another sample on a separate space in the testing card. Kits come with an extra brush, so they should not reuse a used applicator brush. Other kits may require someone to test more than one stool sample. Once done, they should place the appropriate labels and send the sample to the lab for testing.

FIT-DNA test

This test requires an indivual to collect an entire bowel movement in a tube that comes with the kit. A person will then send the sample and kit to a lab for analysis.

A positive fecal occult blood test means that the individual has blood in their stool. This indicates that they are bleeding somewhere in their digestive tract. Having a positive test is not a diagnosis of cancer. Other conditions, such as hemorrhoids, polyps, or IBD, can also cause bleeding. A doctor will typically recommend additional tests, such as a colonoscopy, to help them reach a diagnosis.

A negative test result means that the test did not detect blood in the sample. A doctor may then suggest a person repeats the test in a year. An unclear result may suggest that the test was faulty or that it identified a hint of blood but not enough for a positive result. A doctor will likely suggest that a person repeats the test.

A fecal occult blood test is an important tool that checks for small amounts of blood in the stool. This test can give doctors valuable information about the health of the gastrointestinal tract.

Health experts often use a fecal occult blood test to screen for colorectal cancer, but it can also detect other conditions that cause bleeding. If a person receives a positive result, they should consult their doctor for further investigation.