Stool Tests for Colorectal Cancer
A stool test is one of many tests used to look for colorectal cancer. These tests may find cancer early, when treatment works better. Colorectal cancer affects the large intestine (colon) and the rectum.
There are three kinds of stool tests:
- Fecal occult blood test (FOBT). For this test, you put tiny samples of your stool on a special card or cloth and send it to a lab. The lab uses chemicals to find blood that can't be seen with the naked eye. With some test kits, you can add the chemicals yourself at home. For several days before the test, you can't eat certain foods, and you have to stop taking some medicines.
- Fecal immunochemical test (FIT), also called an immunochemical fecal occult blood test (iFOBT). For this test, you take a sample with a brush and dab it onto a special card. This test may be easier to do at home than the FOBT. There are no drug or food restrictions, and collecting a stool sample may take less effort.
- Stool DNA test (sDNA). Instead of looking for blood in the stool, this test looks for abnormal DNA from cancer or polyp cells. For this test, you collect all of the stool from one bowel movement and put it in a special box that you mail to the lab. There are no drug or food restrictions. Of the three tests, this one has the easiest instructions.
Blood in the stool may be the only symptom of colorectal cancer, but not all blood in the stool is caused by cancer. Other conditions that can cause blood in the stool include:
- Hemorrhoids. These are enlarged, swollen veins in the anus. Hemorrhoids can form inside the anus (internal hemorrhoids) or outside of the anus (external hemorrhoids).
- Anal fissures. These are thin tears in the tissue that lines the anus (anal sphincter) up into the anal canal.
- Colon polyps. These growths of tissue are attached to the colon and often look like a stem or stalk with a round top.
- Peptic ulcers. These sores form when the digestive juices made in the stomach eat away at the lining of the digestive tract.
- Ulcerative colitis. This type of inflammatory bowel disease (IBD) causes inflammation and sores (ulcers) in the inner lining of the colon and rectum.
- Gastroesophageal reflux disease (GERD). This is the abnormal backflow (reflux) of food, stomach acid, and other digestive juices into the esophagus.
- Crohn's disease. This type of inflammatory bowel disease causes inflammation and ulcers that may affect the deep layers of the lining of the digestive tract.
- Use of aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs).
Stool tests may be used to check for colorectal cancer, but they are never used to diagnose it. Other tests for colorectal cancer include flexible sigmoidoscopy, colonoscopy, and CT scan (virtual colonoscopy).
A stool test is one of many tests that may be used to screen for colon cancer. Other tests include sigmoidoscopy, colonoscopy, and computed tomographic colonography. Which screening test you choose depends on your risk, your preference, and your doctor. Talk to your doctor about what puts you at risk and what test is best for you.
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Why It Is Done
Stool tests are done:
- To look for signs of cancer.
- FOBT and FIT/iFOBT tests look for blood in the stool. These tests are useful to screen for colon cancer, because tissue or polyps with cancer are more likely to bleed than normal colon tissue. If bleeding is found, more tests will be done to find the cause. It's important to call your doctor if a home test shows blood in your stool. A home test doesn't replace the need for a regular exam by your doctor.
- Stool DNA tests look for abnormal changes. Colon cancer cells often contain DNA changes in genes. Cells with these changes can be shed into the stool, where this test may be able to find them.
- To help find the cause of abdominal pain.
- To check for the cause of anemia.
- As part of a routine physical exam for those with a higher chance of getting colorectal cancer, especially at age 50 and older.
How To Prepare
Since colorectal cancers do not bleed all the time, FOBT and FIT/iFOBT tests are done over several days on different stool samples. This increases the chance of finding blood in your stool if it exists. A stool DNA test requires only one sample.
You may need to avoid certain foods for 2 to 3 days before the test. This depends on what kind of stool test you use. If you aren't sure, ask your doctor.
Do not do the stool tests during your menstrual period or if you have active bleeding from hemorrhoids. Also, do not test a stool sample that has been in contact with toilet bowl cleaning products that turn the water blue.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).
How It Is Done
For a stool test, it may be helpful to catch the stool on some plastic wrap draped loosely over the toilet bowl and held in place by the toilet seat.
For tests that require more than one sample, be sure to collect each sample the same way.
You will need to collect stool samples over three different bowel movements on three different days. The test kit includes a wooden applicator and test cards. Be sure to follow the instructions that come with your test kit, including any instructions to avoid certain foods in the days before the test.
Other ways to do FOBT:
- Some kits instruct you to use a special cloth to wipe with after a bowel movement. After wiping with the cloth, you put the developer solution on it to check for color change that means there is blood in the stool.
- Other kits have a special test pad that you place in the toilet after having a bowel movement. The pad will change color if the stool has blood in it.
If there is blood in your stool, call your doctor as soon as possible.
You will need to collect two stool samples for testing over different days. The test kit includes brushes and test cards. Be sure to follow the instructions provided in the test kit. The FIT/iFOBT test doesn't require a special diet in the days before you take the test.
Stool DNA test
For this test, you will save all the stool from one bowel movement. You use a special box for the stool and mail it to a lab. You will receive a kit from your doctor with supplies and instructions for doing this test at home. Always follow the instructions in your kit.
How It Feels
You may find it unpleasant to collect a stool sample for these tests.
These tests do not have any risks.
For some FOBT kits, you can read the results yourself. Other tests, including FIT/iFOBT and stool DNA, are read by your doctor.
A normal FIT/iFOBT or FOBT test means that there was no blood in your stool at the time of the test. A normal sDNA test means that no abnormal cells were found. Normal test results are called negative.
An abnormal FIT/iFOBT or FOBT test means that there was some blood in your stool at the time of the test. An abnormal sDNA test means that some abnormal cells were found. Abnormal test results are called positive.
Talk with your doctor about how often you should do a test, depending on your age and any risk factors you may have for colorectal cancer.
A colon polyp, a precancerous polyp, or cancer can cause a positive stool test. With a positive test, there is a small chance that you have early-stage colorectal cancer.
Talk with your doctor about what test you may need next. Most of the time, an abnormal stool test means that you will need to have a colonoscopy.
What Affects the Test
Reasons you may not be able to have a stool test or why the results may not be helpful include:
- Having blood in the urine, menstrual bleeding, hemorrhoids, an anal fissure, bleeding gums, or nosebleeds.
- Having cleaning products in the toilet water at the time of the test.
What To Think About
- Stool tests can produce false-positive and false-negative results.
- False-positive means that the test may be positive when you don't have a polyp or cancer.
- False-negative means that the test may be negative when you do have a polyp or cancer.
- These tests may miss polyps and some cancers.
- The stool DNA test is still new, and it isn't available everywhere.
Other Works Consulted
- Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Haas JS (2013). Adult preventive health care. In EG Nabel, ed., ACP Medicine, section 2, chap. 2. Hamilton, ON: BC Decker.
- Hoffman RM, et al. (2010). Colorectal cancer screening adherence is higher with fecal immunochemical tests than guaiac-based fecal occult blood tests: A randomized, controlled trial. Preventive Medicine, 50(5–6): 297–299.
- Levin B, et al. (2008). Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: A joint guideline from the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA: A Cancer Journal for Clinicians, 58(3): 130–160.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
|Adam Husney, MD - Family Medicine|
|Arvydas D. Vanagunas, MD - Gastroenterology|
|Last Revised||October 30, 2013|
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