How Do Stool Tests for Colon Cancer Work?

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    How Do Stool Tests For Colon Cancer Work? 


    Colorectal cancer, or colon cancer, occurs when abnormal cells in the colon or the rectum begin to divide uncontrollably and form a malignant tumor. Colon cancer often begins as a very small lesion or growth in the lining that covers the surface of the colon or rectum. These lesions, or polyps, can appear raised, flat, or indented. Colorectal polyps typically occur in people who are older than the age of 50. These abnormal growths of tissue are not always cancerous, but some are more likely to become cancerous – these are adenomas. There are several screening tests that have been developed to detect the presence of abnormal tissue in the colon and rectum, including at home colon cancer screening tests or stool tests. 

    Polyps and colorectal cancer tumors can bleed resulting in blood in the stool, or feces. An at home colon cancer stool tests is often prescribed to low-risk for colon cancer patients to check for small amounts of blood in stool that are not visible with the naked eye. There are three types of stool tests currently approved by the FDA to screen for colorectal cancer: 

    • Stool guaiac tests or gFOBT– Uses a chemical to detect blood proteins present in hemoglobin
    • FIT (fecal immunochemical test)– Uses an antibody to detect the protein hemoglobin in blood
    • FIT-DNA test (Cologuard®️) – Uses an antibody to detect hemoglobin and detects DNA biomarkers that indicate that colorectal cancer is present. 

    All three tests are simple to perform. You need to gather a stool sample from home according to each kit’s specific directions and then mail the sample to a lab for analysis. You may be curious about how the labs detect abnormal cancer cells in the sample and wonder who interprets the results. Each test is unique once the sample reaches the lab in how the scientists test the sample for abnormalities. Read further to gain an understanding of how the FOBT, FIT, and Cologuard ®️ tests work from sample collection to its analysis in the lab and who interprets the results. 

    Guaiac Stool Tests 

    The guaiac fecal occult blood test (gFOBT) detects hidden blood in feces. When a person has a polyp or cancerous tumor in the colon, the surfaces of these growths will bleed because the blood vessels at the surface are fragile and are easily broken open when feces pass. These broken blood vessels will leak blood into the feces but the amount is so small it is rarely seen with the naked eye. In a fecal occult blood test, the chemical guaiac is used to detect the presence of these microscopic amounts of blood. 

    The guaiac fecal occult blood test (gFOBT) uses a chemical to detect the presence of blood components found in the hemoglobin protein of our red blood cells. Hemoglobin is the protein in red blood cells that carries oxygen to the body’s cells and transports carbon dioxide away from the body’s organs and tissues back to the lungs. Currently, there are two stool guaiac tests that detect the presence of these blood proteins in a stool sample:

    • High-sensitivity guaiac-based fecal occult blood test (HS-gFOBT)
    • Low-sensitivity guaiac-based fecal occult blood test  (LS-gFOBT)

    The low-sensitivity gFOBT uses the chemical guaiac to detect heme, the iron-containing protein that is found in hemoglobin responsible for binding oxygen in the bloodstream. 

    The high-sensitivity gFOBT also uses guaiac to detect the heme protein but has an added enhancer that reduces the number of false-positives from substances in the stool sample that mimic the heme protein. 

    How do stool guaiac tests work?

    The test is completely noninvasive and only requires you take a sample of your stool after you use the bathroom. You will need to take samples from three separate bowel movements. After you follow the instructions with your kit you will mail the sample envelope to the lab to be analyzed. 

    Once the sample reaches the lab, your stool will be analyzed for blood using a simple chemical reaction. Guaiac is an antioxidant chemical that is extracted from the wood resin of the Guaiacum tree native to the Bahamas. Chemists use this chemical to detect microscopic amounts of blood by a simple reaction involving hydrogen peroxide.This is how it works: 

    1. The stool sample is placed on a paper that contains guaiac. 
    2. Hydrogen peroxide is dripped onto the paper. 
    3. The paper will rapidly turn blue if the heme protein from blood is present.
    4. No color change indicates there is no blood in the sample.

    Guaiac reacts not only with heme but with chemicals found in plants. People who are taking the test have to abstain from iron supplements, red meat, certain vegetables, vitamin C, and citrus fruit for several days before the test. 

    Note: The low-sensitivity gFOBT test has generally been replaced by the more accurate high-sensitivity gFOBT test FIT screening test or the FIT screening test. Both have a higher degree of sensitivity for detecting colorectal cancer. 

    Fecal Immunochemical Test (FIT)

    Like the guaiac stool tests, the fecal immunochemical tests examine stool samples for the presence of microscopic amounts of blood that cannot be seen with the naked eye. FIT is considered a newer type of fecal occult blood test and is sometimes referred to as iFOBT or immunochemical fecal occult blood test.

    Instead of using a chemical to detect the presence of blood, FIT uses antibodies to detect the blood protein globin found in hemoglobin. These tests are designed to detect specifically human globin proteins and will not react with blood proteins from non-human blood that is found in red meats. People who take the FIT to screen for colorectal cancer do not need to avoid red meats and other foods or medications before they take the test. 

    How does FIT work?

    The FIT is done the same as the guaiac stool test, except without the dietary or drug restrictions. You will collect a stool sample according to the directions on the kit and mail your sample to the lab listed on your kit. Once the sample reaches the lab it is sent to a pathologist who will test the stool sample with antibodies for the presence of the globin protein. 

    The lab component of the fecal immunochemical test is more complicated than the simple color-changing guaiac stool tests. In the lab, the sample is analyzed by using antibodies that specifically attach to the globin protein component of the hemoglobin molecule. If the antibodies bond with the blood protein then the pathologists know blood is present in the stool sample. Scientists use a number of different methods to measure the amount of blood that is present in the sample. The test is highly sensitive to low concentrations of globin and does not interfere with medications or foods.

    FIT-DNA Test (Cologuard)

    The lining of the colon sheds cells routinely throughout the day. This shedding allows the intestinal lining to renew and maintain its healthy tissue for optimal health. These epithelial cells end up in the feces and are often examined by pathologists to test for diseases such as colorectal cancer. Abnormal cells from a polyp or tumor will also shed into the feces along with any occult blood caused by broken blood vessels. Cancer cells will have altered DNA fragments that can be detected with special lab tests like the FIT- DNA test or Cologuard. The Cologuard home colon cancer screening test detects cancer by identifying DNA markers from abnormal cells and occult blood in stool samples. 

    How does Cologuard work?

    Like all stool tests, you will be asked to collect a stool sample during a regular bowel movement. There are no dietary or medication restrictions you need to follow before collecting your sample. The testing kit usually contains labels, a sample container, a tube, preserving liquid, and instructions for you to follow. Once you collect your sample, you will ship it to the lab within 24 hours. 

    The Cologuard screening test is a combination of two tests, the fecal immunochemical test and a DNA screening for abnormal cancer cells. Lab pathologists will test your stool sample for increased levels of mutated or altered DNA biomarkers that are found in shedded colon cells. These biomarkers indicate the presence of colorectal cancer or precancerous tumors known as adenomas. Your sample will also be tested for the presence of the globin protein found in blood using the antibody test, fecal immunochemical test (FIT).  

    Who reads the results of my stool tests for colon cancer?

    If your sample was sent to a lab, pathologists will interpret the results and send them back to your physician within two weeks. 


    American Cancer Society Writing Staff. (2019). Clinicians reference: Stool-based tests for colorectal cancer screening. Retrieved from NCCRT:

    Redwood, D., et., al. Comparison of fecal occult blood tests for colorectal cancer screening in an alaska native population with high prevalence of helicobacter pylori infection, 2008–2012.Preventing Chronic Disease. Retrieved from CDC:

    Shapiro, J. A., et., al. Comparison of fecal immunochemical and high-sensitivity guaiac tests for colorectal cancer screening. American Journal of Gastroenterology, 112(11), 1728-1735. Retrieved from PubMed:

    Shaukat, A., et., al. (2021, March). ACG clinical guidelines: Colorectal cancer screening 2021. American Journal of Gastroenterology,116,3,458-479. Retrieved from PubMed: