At a Glance
Why Get Tested?
To screen for digestive tract bleeding, which may be an indicator of colon cancer
When to Get Tested?
Annually, beginning at age 50 as part of a routine examination to screen for colon cancer; as directed by your doctor based on family medical history
The American Cancer Society recommends that three stool samples should be collected over several days and prepared on an occult blood card in order to provide for the most effective screening.
Test Preparation Needed?
For the guiac-based FOBT (gFOBT), you will be instructed to avoid certain medications and follow certain dietary restrictions for several days before collecting the stool samples. For immunochemical FOBT, there are no dietary or drug restrictions.
The Test Sample
What is being tested?
The fecal occult blood test (FOBT) or fecal immunochemical test (FIT) checks for hidden blood in the stool. Normally, only very small amounts of blood may be lost from the stomach or intestines during digestion. This blood cannot be seen in the feces and does not produce a positive result on a fecal occult blood test.
However, polyps, finger-like growths that protrude into the intestinal cavity (lumen) or in the rectum, can be fragile and bleed intermittently, such as when food waste brushes against them. The blood released is not usually visible in the stool but can be detected with a fecal occult blood test. Benign polyps are relatively common in people over the age of 50 but can become cancerous and potentially spread to other parts of the body (metastasize). Most cases of colon cancer begin with the development of benign intestinal polyps. As a result, blood in the stool may indicate the presence of these polyps, which, if not attended to, may progress to cancer. Often, this small amount of blood is the first and sometimes the only sign of early colon cancer, making the fecal occult blood test a valuable screening tool for colorectal (colon and rectal) cancer.
There are a few different methods that can be used for fecal occult blood testing:
- The guaiac smear method (gFOBT)—this method uses a chemical indicator that shows a color change in the presence of blood. It is recommended that the newer, high-sensitivity tests be used.
- An over-the-counter (OTC) flushable reagent pad/tissue method
- Immunochemical method (iFOBT or FIT)—this method uses antibodies directed against human hemoglobin to detect blood in the stool.
Regardless of choice of method, it is recommended that the test be performed on at least three stool samples collected on different days. The American Cancer Society has stated that a single test performed at the time of a digital rectal exam in a doctor's office is not recommended because it may not be sensitive enough to screen for cancer. The home FOBT or FIT is recommended because the collection of stool on three different days increases the chance of detecting cancer. Furthermore, it is recommended that those who choose this method of colon cancer screening get screened every year.
How is the sample collected for testing?
Each method has a different approach to collecting and testing stool samples for occult blood.
- For the guaiac-based test (gFOBT), the doctor or laboratory will typically provide three test cards. Separate stool samples are collected from different bowel movements, usually on three consecutive days. For each test, a stool sample should be collected into a clean container and should not be contaminated with urine or water. A test card is labeled with the person's name and the date; then, with an applicator stick, a thin smear of stool is put onto a designated area on the card and allowed to dry. Once it is dry, it is stable for several weeks at room temperature. When all of the consecutive samples have been collected and dried, the test cards are returned to the doctor/laboratory, usually by mailing them.
- With the OTC flushable reagent pad/tissue method, a test pad/tissue is placed in the toilet after a bowel movement. The pad contains a chemical that produces a color change when blood is present. The person doing the test watches for the characteristic color change and records the findings on a report form. Like the gFOBT, this test is usually done on three consecutive days and then the completed form is returned to the doctor. Be aware that use of toilet bowl cleansers or the presence of blood derived from urine or a woman's menstrual period may alter results.
- With the immunochemical tests, the collection method may vary based on the specific manufacturer. A common approach is to use a special long-handled brush or other device to collect a sample from the surface of a stool sample. The brush or device is then used to transfer the sample to a special collection card. After the card is allowed to dry, it is returned to the doctor or laboratory. At least three such samples are collected on different days and sent in one mailing.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
For guaiac-based FOBT and OTC flushable methods, there are special dental, dietary and drug restrictions. These tests detect any blood that enters the digestive tract. Therefore, steps that are taken to avoid introducing blood into the digestive tract will increase the quality of the test sample.
- Blood that arises from bleeding gums (following dental procedures or gum disease) may be detected by these tests. You should avoid having any dental procedures up to three days before beginning to collect stool samples.
- Bleeding in the stomach that may be triggered by use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, naproxen, and ibuprofen. Avoid taking these drugs for seven days prior to testing, if clinically possible.
- The guaiac-based gFOBT and the OTC flushable pad/tissue rely on a chemical reaction to produce the color change that gives a positive test. Foods such as red meat, broccoli, turnips, cauliflower, apples, oranges, mushrooms, and horseradish, and drugs such as colchicine and oxidizing drugs (like iodine and boric acid) may also trigger the same chemical reaction and make the test appear positive even in the absence of human blood (a false positive result). You may be instructed to avoid these foods and drugs three days prior to and during the testing period.
- Vitamin C, on the other hand, interferes with the chemical reaction and prevents the color formation that should occur when blood is present (a false negative result). Vitamin C supplements and fruit juices that contain vitamin C should be avoided three days prior to and during testing.
Follow the instructions that are provided by the doctor or included in test kit instructions. Check with the doctor before stopping any drugs to be certain that it is safe to do so.
For immunochemical methods, there are no dietary, drug, or dental procedure restrictions. The test uses antibodies to detect only human blood from the lower digestive tract (colon).
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
(Revised 2011 June 20). Colorectal Cancer Overview. American Cancer Society [On-line information]. Available online at http://www.cancer.org/Cancer/ColonandRectumCancer/OverviewGuide/index through http://www.cancer.org. Accessed July 2011.
(Revised 2011 June 24). Colorectal Cancer Early Detection. American Cancer Society [On-line information]. Available online through http://www.cancer.org. Accessed July 2011.
(Updated 2009 July 31). Colorectal Cancer. CAP [On-line information]. Available online through http://www.cap.org. Accessed July 2011.
Eisner, T. (Updated 2010 November 12). Stool guaiac test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003393.htm. Accessed July 2011.
Dugdale, D. (Updated 2011 February 16). Flushable reagent stool blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/007007.htm. Accessed July 2011.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby’s Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 913-915.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 782-785.
(March 5, 2008) Levin B, et al, Screening and Surveillance for the Early Detection of Colorectal Cancer and Adenomatous Polyps, 2008: A Joint Guideline from the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA Cancer J Clin 2008. Available online at http://caonline.amcancersoc.org/cgi/content/full/CA.2007.0018v1 through http://caonline.amcancersoc.org. Accessed July 2011.
Dreyfuss J. Fecal Occult Blood Testing Has Great Potential as a Screening Tool for Colorectal Cancer. CA Cancer J Clin 2010; 60:275-276. Available online at http://caonline.amcancersoc.org/cgi/content/full/60/5/275 through http://caonline.amcancersoc.org. Accessed July 2011.
Fraser C. Fecal Occult Blood Tests Life Savers or Outdated Colorectal Screening Tools? Clinical Laboratory News, March 2011: Volume 37, Number 3. Available online at https://www.aacc.org/publications/cln/2011/march/Pages/FecalOccult.aspx through https://www.aacc.org. Accessed July 2011.
Sources Used in Previous Reviews
American Cancer Society. Detailed Guide: Colon and Rectum Cancer. Can Colorectal Polyps and Cancer Be Found Early? Colorectal Cancer Screening. Available online through http://www.cancer.org. Accessed 2/4/08.
Laurie Barclay, MD (News author); Hien T. Nghiem, MD (CME author). Fecal Immunochemical Test May Be Most Effective for At-Home Colon Cancer Screening (CME). Release Date: November 15, 2006. Available online through http://www.medscape.com. Accessed February 2008.
US Centers for Disease Control and Prevention. Colorectal cancer screening guidelines. Reviewed/updated 16 Feb 2007. Available online at http://www.cdc.gov. Accessed 15 Feb 2008.
Bernard Levin, MD, et al. Screening and Surveillance for the Early Detection of Colorectal Cancer and Adenomatous Polyps, 2008: A Joint Guideline from the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA Cancer J Clin 2008. Available online through http://caonline.amcancersoc.org. Accessed February 2008.
American Cancer Society. Press release: Health Groups Issue Updated Colorectal Cancer Screening Guidelines; Recommendations Include Two New Tests and Preference for Tests that Detect Precancerous Polyps. Atlanta 2008/03/05. Available online through http://www.cancer.org. Accessed February 2008.
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Kirchner, J. (2001, June 1). Fecal Occult Blood Testing to Screen for Colon Cancer. American Family Physician [On-line serial]. Available online through http://www.aafp.org.
Apgar, B. (2000, March 15). Guaiac Examination of the Rectum for Fecal Blood. American Family Physician [On-line serial]. Available online through http://www.aafp.org.
CDC (2001, March 9). CDC says colorectal cancer screening rates remain low. Centers for Disease Control [Press Release]. Available online through http://www.cdc.gov.
MedlinePLUS (Page updated 2002, January 2). Fecal occult blood test (FOBT). MedlinePLUS Encylopedia [On-line information]. Available online through http://www.nlm.nih.gov/medlineplus/ency/article/007008.htm.
AGA (2001). Clinical Practice Recommendations -- People at Average Risk. The American Gastroenterological Association [On-line information]. Available online through http://www.gastro.org.
NCI (2001, August 23). Questions and Answers about Screening, Early Detection, and Treatment for Colorectal Cancer. National Cancer Institute, Cancer Facts [On-line information]. Available online through http://cis.nci.nih.gov.
Levin, B. et al. Emerging Technologies in Screening for Colorectal Cancer. CA Cancer J Clin 2003; 53:44-55. Available online through http://caonline.amcancersoc.org.
CMS: Medicare Announces Intention to Cover Screening Immunoassay Fecal-Occult Blood Tests; November 5, 2003. Available online through http://www.cms.hhs.gov.