What is a Fecal Occult Blood Test?

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  • Written By: Lucinda Reynolds
  • Edited By: W. Everett
  • Last Modified Date: 14 August 2019
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The fecal occult blood test is a specific lab test used to find blood in feces. The term occult means hidden. This test is sometimes referred to as a stool quaiac test. A positive test result could be an early indicator of colorectal cancer. This is why many physicians rely on this test to give them an idea of when further testing may be necessary.

Fecal occult blood testing is easy to do. Many individuals will be able to perform this test at home with a kit supplied by the doctor. A small sample of stool is obtained using a wooden applicator. The stool is smeared onto a test card, and the card is then sealed and taken back to the doctor's office. Usually, the doctor will want to obtain three stool guaiac tests from three different stool samples.

Once the doctor receives the sample card, he will apply a developing agent to the sample. If the agent turns blue, it means there may be blood present. It is possible for these tests to detect blood in small amounts. The test will begin to turn positive if you have as little as 2 teaspoons (10 ml) of blood in your feces.


Colorectal cancer is not the only condition that can cause a positive result when using this type of blood test. Individuals who have bleeding ulcers or diverticulitis can test positive for blood in the stool. A fecal occult blood test is not able to diagnose of cancer. It is merely a tool to indicate there may be a problem.

Before an individual performs a fecal occult blood test, he should avoid red meat and vitamin C supplements. These two things can sometimes cause a false positive result. There also are certain foods that can produce a false positive. Beets, carrots, and cauliflower are just a few foods that can make the test be falsely positive.

An individual should not perform stool occult blood testing if she has diarrhea or is on her period. Constipation and hemorrhoids flare ups would be reasons to delay testing also. These conditions are all minor problems, but they can produce small amounts of blood in the stool.

Certain medications such as iron supplement and bismuth products can make stools turn black. Black stools are a sign of upper gastrointestinal bleeding. Performing stool occult blood testing can help determine if the black stools are caused by bleeding or by medicines.

It is recommended that individuals over age 50 have a yearly fecal occult blood test. This should be a routine screening much like a yearly pap smear or mammogram. In most cases, when cancer is detected early, the chances are much better for long term survival.


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