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Fecal Occult Blood Test Types: Guaiac vs. Immunochemical

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    Written by Alissa Daschbach MA BS BA

    Colon cancer screening tests typically are used to detect or identify the presence of polyps or cancerous tumors in an effort to catch colon cancer before it spreads. While a colonoscopy is considered the gold standard for detecting polyps and colorectal cancer, many physicians will prescribe at home colon cancer tests like the fecal occult blood test to screen for colon cancer.

    FIT and FOBT have a high level of sensitivity for detecting colon cancer through the detection of hidden blood in the stool

    Fecal Occult Blood Test Types: Guaiac vs. Immunochemical

    A polyp is a small growth that is found in the colon that can become cancerous with time depending on its cellular makeup. These abnormal growths contain blood vessels at the surface that are fragile and 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. Fecal occult blood tests detect these microscopic amounts of blood using laboratory techniques. There are two main types of fecal occult blood tests:

    • Fecal immunochemical test (FIT) – This test uses antibodies that specifically react with a blood protein in the stool
    • Chemical-based guaiac fecal occult blood test (gFOBT) – The FOBT uses the chemical guaiac to detect microscopic amounts of blood in the stool 

    Both the FIT and gFOBT tests detect hidden blood in stool samples that are collected at home. However, the nature of the laboratory analysis is distinct between the two along with the dietary preparation before the test.

    Both tests detect hidden blood in stool samples that are collected at home. However, the nature of the laboratory analysis is distinct between the two along with the dietary preparation before the test. Read further to learn about the differences between the FIT and FOBT, including a comparison of accuracy, cost, and procedures.

    GFOBT and FIT: How Are They Different? 

    Both the guaiac and immunochemical fecal occult blood tests detect hidden blood in the stool collected by patients using kits used in their homes. Each test has a unique set of directions that must be followed to assure that the stool is collected properly and there is enough sample for the pathologists to conduct their analysis. The main differences between the gFOBT and FIT include:

    • FIT does not require dietary or medication restrictions before collecting the sample
    • gFOBT requires patients to restrict diet and some medications before stool collection
    • FIT asks for a single collection of stool sample
    • gFOBT asks for three separate stool samples to be collected

    Guaiac Stool Tests 

    The guaiac fecal occult blood test (gFOBT) uses the chemical guaiac to detect the presence of blood components found in the heme protein of our red blood cells. The most current version of the guaiac stool test is known as the high-sensitivity gFOBT due to its ability to specifically detect the heme protein found in human blood only. 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.

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

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

    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. 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. The test is highly sensitive to low concentrations of globin and does not interfere with medications or foods.

    Accuracy 

    Fecal occult blood tests are graded on accuracy using two rating factors: sensitivity and specificity. A sensitivity rating indicates the percent likelihood that the stool test will detect hidden blood in the stool. On the other hand, a specificity rating indicates the percent likelihood that there is no detectable blood present in the stool. A test with a high sensitivity is less likely to give a patient a false negative result, while a test with a high specificity will give a low number of false positive results.

    These two rating factors help to describe how valid or accurate a test is. The accuracy of the gFOBT and FIT tests according to different at home colon cancer test providers are as follows:

    FIT Providers

    Second Generation FIT® Colon Cancer Test

    Sensitivity 98%

    Specificity 96% 

    LetsGetChecked® Colon Cancer Screening Test

    Sensitivity 79%

    Specificity 94% 

    Pixel by LabCorp® Colorectal Cancer At-Home Test

    Sensitivity 74%

    Specificity 96% 

    FOBT Providers

    EZ Detect Stool Blood Test

    Sensitivity 41%

    Specificity 97%

    Hemmocult ( includes Hemoccult® II SENSA)

    Sensitivity 74%

    Specificity 98.6%

    Procedural Differences Between the FOBT and FIT

    The major difference between the procedures of an FOBT and an FIT is the preparation for the tests. In both tests, you will be collecting a stool sample and mailing it in a secure package back to a lab for analysis. However, the chemical-based nature of the FOBT requires patients to monitor their diet and medications and restrict those that can cause a false positive or false negative result. The immunochemical fecal occult blood test (FIT) does not require the patient to restrict their diet or medications. The other major difference is that the FIT only requires one stool sample to be collected while the gFOBT asks for three separate stool samples to be collected. 

    Guaiac Fecal Occult Blood Test (gFOBT) Preparation and Procedure

    Preparation

    The fecal occult blood test, or FOBT, is a chemically sensitive test and does require you to restrict your diet a couple days before the test. This is because certain foods and medications can trigger false positive or false negative results. A false positive result means that a person receives a positive result for having blood in the stool when in reality there is no blood. Conversely, a false negative result means that the test missed detecting blood in your stool that otherwise should have been detected. Many foods and some vitamin supplements can interfere with this reaction and are listed below.

    Drugs and substances

    • Aspirin
    • Boric acid
    • Bromides
    • Colchicine
    • Iodine
    • Nonsteroidal anti inflammatory drugs (NSAIDs)

    Foods and supplements

    • Red meat
    • Beets
    • Carrots
    • Cauliflower
    • Mushrooms
    • Broccoli
    • Radishes
    • Turnips
    • Horseradish
    • Cantaloupe
    • Iron supplement
    • Vitamin C 

    Procedure

    The gFOBT asks for three different samples that are smeared on a slide that is labeled in three different windows. The kit contains everything you need to perform the test, including the envelope to return the sample.

    1. Open flap of slide.
    2. Sit on the toilet like you normally do to have a bowel movement.
    3. Use the clean, dry container to catch your stool before it touches the water.
    4. Collect a sample of your stool using one end of the applicator stick.
    5. Thinly smear the sample inside the square (usually marked with “A”).
    6. Throw away the stick into the trash.
    7. Close the cover of the slide.
    8. Repeat procedure two more times with a stool sample and smear in windows (usually marked “B” and “C”). 

    Immunochemical-based fecal occult blood test (FIT)

    Preparation

    There are no food or medications restrictions for this test. You are able to follow your normal daily routine, including eating your regular diet and taking your prescribed medications. 

    Procedure

    Basically, you are simply collecting a small sample of your stool into a sterile tube after you have a bowel movement. The kit you have contains everything you need to perform the test, including the envelope to return the sample. The steps to this test typically are as follows:

    1. Flush the toilet before your bowel movement.
    2. If provided, place paper in the toilet to catch bowel movement.
    3. Use the brush provided by the kit to brush the surface of your stool and dip into toilet water.
    4. Touch brush with sample on the space indicated on test paper.
    5. Place the sample in the provided container and mail the sample in according to directions. 

    Cost Differences Between Providers

    Immunochemical Fecal Occult Blood Tests (FIT)

    Everlywell FIT® Colon Cancer Screening Test

    Cost

    One kit: $25.00

    Two tests: $30-40

    LetsGetChecked® Colon Cancer Screening Test

    Cost

    One kit: $69

    Pixel by LabCorp™ Colorectal Cancer At-Home Test  

    Cost

    One kit: $99 

    Guaiac-based Fecal Occult Blood Tests (gFOBT) 

    Hemoccult II 

    Cost

    One kit: Available by prescription only $25

    Clarity   

    Cost

    One kit: Available by prescription only $25

    References

    Ancrum, S. (2020, August 10). Everylywell review: Pros, cons, and how it works. Retrieved from Farr Institute: https://www.farrinstitute.org/everlywell-review/

    Cooper, L. (2016, December 01). At-home colon cancer tests: What you need to know. Retrieved from Consumer Reports: https://www.consumerreports.org/colonoscopy/at-home-colon-cancer-tests/

    Cancer Care Ontario Writing Staff.  (2021). Fecal Immunochemical Test (FIT) Instructions. Retrieved from Cancer Care Ontario: https://www.cancercareontario.ca/en/types-of-cancer/colorectal/screening/fit-instructions

    Cancer Care Ontario Writing Staff.  (2021). FIT Instructions Pamphlet. Retrieved from Cancer Care Ontario: https://www.cancercareontario.ca/sites/ccocancercare/files/assets/FITKitInstructions.pdf

    Kaur, K., & Adamski, J. J. (2020, August 16). Fecal Occult Blood Test. Retrieved from StatPearls: https://www.ncbi.nlm.nih.gov/books/NBK537138/

    Memorial Sloan Kettering Cancer Center. (2020, August 04). How to do a fecal occult blood test (FOBT). Retrieved from MSKCC: https://www.mskcc.org/cancer-care/patient-education/fecal-occult-blood-test

    Rabeneck, L., et., al. (2012, March 01). Fecal immunochemical tests compared with guaiac fecal occult blood tests for population-based colorectal cancer screening. Canadian Journal of Gastroenterology, 26(3), 131-147. Retrieved from Europe PMC: http://europepmc.org/article/PMC/3299236

    Watson, R. (2020, August 03). Cologuard for cancer screening: What you need to know. Retrieved from Healthline: https://www.healthline.com/health/cologuard

    U.S. National Library of Medicine. (2020, July 31). Fecal occult blood test (FOBT). Retrieved from MedlinePlus: https://medlineplus.gov/lab-tests/fecal-occult-blood-test-fobt/

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