Fecal Occult Blood Test and Fecal Immunochemical Test
Also known as:FOBT; Stool Occult Blood; Hemoccult; Guaiac Smear Test; gFOBT; Immunochemical FOBT; Immunoassay FOBT; iFOBT; FIT
Formal name:Occult Blood Test, Fecal
Related tests:Complete Blood Count, Calprotectin, Stool DNA
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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 healthcare provider based on your family medical history
Three stool samples are typically collected over several days and prepared on occult blood cards 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 the fecal immunochemical test (FIT), there are no dietary or drug restrictions.
How is it used?
The main use for the fecal occult blood test (FOBT) or the fecal immunochemical test (FIT) is as a screen for early colon cancer. Most cases of colon cancer begin with the development of benign intestinal polyps. Benign polyps are relatively common in people over the age of 50. Most are harmless, but some can become cancerous.
Blood in the stool may be the only symptom of early cancer; thus, if detected early, treatment can begin immediately, improving the chance of a cure.
Methods of testing for fecal occult blood include:
- The guaiac smear method (gFOBT) – uses a chemical indicator that shows a color change in the presence of blood
- An over-the-counter (OTC) flushable reagent pad/tissue method also produces a color change in the presence of blood.
- Immunochemical method (iFOBT or FIT) – uses antibodies directed against human hemoglobin to detect blood in the stool
A secondary use of FOBT is to determine the cause of anemia, such as blood loss from a bleeding ulcer. If someone has signs and symptoms of anemia, such as fatigue, a low hemoglobin and hematocrit, and/or unusually dark stools, a health practitioner may order the FOBT.
When is it ordered?
The American Cancer Society (ACS) and other major healthcare organizations recommend yearly testing when a person chooses fecal occult blood testing as the method of screening for colon cancer. The ACS and others advise that screening for colon cancer begin at age 50 for the general population, but it may begin at an earlier age when a person has a family medical history of colon cancer.
A health practitioner may sometimes order an FOBT when someone has unexplained anemia that might be caused by bleeding in the digestive tract.
What does the test result mean?
The fecal occult blood test is normally negative.
For the guaiac-based FOBT, a positive test result indicates that abnormal bleeding is occurring somewhere in the digestive tract. This blood loss could be due to ulcers, diverticulosis, polyps, inflammatory bowel disease, hemorrhoids, blood swallowed due to bleeding gums or nosebleeds, or benign or cancerous tumors.
For the fecal immunochemical test (FIT), a positive result indicates abnormal bleeding in the lower digestive tract. Since this test detects only human hemoglobin, other sources of blood, such as from the diet, do not cause a positive result. Moreover, hemoglobin from bleeding in the upper digestive tract is broken down before it reaches the lower digestive tract and is not detected by the FIT. Thus, the FIT is a more specific test than gFOBT.
A positive result from either the guaiac-based FOBT or immunochemical FIT requires follow-up testing. This usually involves direct imaging of the colon and rectum (sigmoidoscopy or colonoscopy).
Is there anything else I should know?
Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples. Taking three different samples on three different days increases the chances of detecting bleeding that is intermittent.
What is being tested?
Most cases of colon cancer begin with the development of benign intestinal polyps. Benign polyps are relatively common in people over the age of 50, and while most do not cause health problems, some can become cancerous and potentially spread to other parts of the body (metastasize). These finger-like growths protrude into the intestinal cavity (lumen) or the rectum. They 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 it can be detected with a fecal occult blood test (FOBT) or fecal immunochemical test (FIT). This small amount of blood may be the first and sometimes the only sign of early colon cancer, making the FOBT and FIT valuable screening tools for colorectal (colon and rectal) cancer. Methods for testing include a guaiac-based test (gFOBT), an over-the-counter (OTC) flushable reagent pad, and an immuochemical method (iFOBT or FIT).
It is recommended that testing 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 health practitioner'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 health practitioner 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 health practitioner or 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 health practitioner. 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 fecal immunochemical tests (FIT), 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 health practitioner 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. Avoid having any dental procedures up to three days before beginning to collect stool samples.
- Bleeding in the stomach 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). If instructed to do so, 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 your health practitioner or included in test kit instructions. Check with your health practitioner before stopping any drugs to be certain that it is safe to do so.
For fecal immunochemical test (FIT) 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).
- Other than stool tests, are there other ways of screening for colon cancer?
Yes. There are imaging tests that may be used to prevent and/or screen for colon cancer. One of these procedures may be chosen instead of screening annually with a fecal occult blood test:
- Sigmoidoscopy is an examination of the rectum and lower colon with a lighted instrument. If polyps are found, they may be removed during the procedure and examined by a pathologist to see if cancer is present. If this is the chosen method of screening, once every 5 years is recommended.
- Colonoscopy is a more thorough examination of the rectum and entire colon using a flexible tube. It also allows for the removal of any polyps. This procedure is recommended for screening every 10 years.
- CT colonoscopy (virtual colonoscopy) is a less invasive procedure that uses computed tomography to visualize the entire colon. The recommended screening interval is 5 years.
- Double contrast barium enema is a series of x-rays of the colon and rectum. It is recommended every 5 years.
- Is the over-the-counter (OTC) at-home test just as good as other stool tests?
While the OTC tests that are dropped into the toilet are fairly sensitive, your health practitioner may prefer the gFOBT or FIT. The color change that is observed in OTC tests is subject to interpretation by the patient. Laboratory professionals are more experienced than most people in interpreting the sometimes subtle color change and are more familiar with sources of false-negative and false-positive results.
- What kind of procedures might follow a positive FOBT or FIT?
An FOBT or FIT may be preceded or followed by a digital rectal exam to detect any growths that may be present in the rectum. A positive FOBT or FIT may be followed by sigmoidoscopy or colonoscopy, which enables the health practitioner to see inside the colon and identify polyps or other abnormalities (see Question 1).
- Are there any other types of tests for colorectal cancer that are run on stool samples?
Yes. There is a stool DNA (sDNA) test available. The test can detect certain alterations to DNA (genetic material) in cancer cells that are shed into the stool. Studies have shown that the test has acceptable sensitivity, although it cannot identify all types of colorectal cancer. Guidelines released in March 2008 jointly by the American Cancer Society, the American College of Radiology, and the US Multi-Society Task Force on Colorectal Cancer added sDNA to their list of recommended testing options for colorectal cancer screening. The American Cancer Society recommends that if this test method is used, it should be repeated every 3 years.
© 2017 American Association for Clinical Chemistry, republished from Lab Tests Online.*
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