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.
Stool tests include:
- Fecal immunochemical test (FIT).
- Fecal occult blood test (FOBT).
- Stool DNA (sDNA/Cologuard).
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).
A stool test is one of many tests that may be used to screen for colon cancer. Other tests include sigmoidoscopy, colonoscopy, and CT 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 blood in the stool, since cancer in the colon or polyps in the colon are more likely to bleed than normal colon tissue. One type of stool test, the sDNA test, checks the stool for blood and genetic changes in DNA that could be signs of cancer. If blood or DNA changes are found, more tests, such as a colonoscopy, will be done to find the cause.
How To Prepare
Since colorectal cancers do not bleed all the time, some stool tests are done over several days on different stool samples. This increases the chance of finding blood in your stool if it exists.
With the FOBT, you will be given instructions about foods to avoid in the days before the test. Some medicines may also need to be stopped for a brief time before the test.
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
There are different types of home tests. The companies that make the test kits provide instructions. To get accurate results, carefully follow the instructions in your kit.
Fecal immunochemical test (FIT)
The test kit contains the things you need to collect small samples of stool. For some types of FIT, you may need to collect a stool sample on 2 or more days.
The FIT test doesn't require a special diet in the days before you take the test.
When the test is done, follow the instructions to return the test. Some tests provide the results right away. If your test shows that blood was found, call your doctor as soon as possible.
Fecal occult blood test (FOBT)
The test kit contains the things you'll need, such as test cards or a special test pad. You may need to collect stool samples over three different bowel movements on three different days. Be sure to follow any instructions about foods or medicines to avoid in the days before the test.
Your instructions may say to:
- Put tiny samples of stool onto paper cards. Return all the samples right after you collect the last sample.
- Place a special test pad 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.
Stool DNA test (sDNA/Cologuard)
The test kit has a container for you to collect an entire stool sample to send to the lab.
The kit includes instructions that show how to collect a stool sample. You may also be asked to take a small sample from the stool that you collected and seal it into a small plastic tube. The kit contains a bottle of liquid preservative that you will pour into the container to cover the stool before sealing the lid. Follow the instructions for sending the kit to the lab.
There is no risk from the stool test itself. For this test, you put a sample of stool on a card or you collect a stool sample. Or you may collect the entire stool.
But there are some important things to think about. If your test is positive, you will need to have a colonoscopy. This would be used to see if the stool test result is from colorectal cancer. But blood in the stool is more often caused by something other than cancer. These other causes could include hemorrhoids, ulcers, or taking aspirin.
Stool test results that are positive when you don't have cancer are called false-positive test results.
Your chances of getting a false-positive test result are higher with the sDNA test compared to FIT or FOBT. This means that you may be more likely to have a colonoscopy that you may not need if you do the sDNA test.
If your test sample is sent to a lab or returned to your doctor's office, your test results will likely be read by your doctor. Some labs may send you the results. And depending on the type of test you choose, you may be able to see the results after completing the last step.
A normal FIT 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 blood or changed genes were found. Normal test results are called negative.
An abnormal FIT 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 blood or changed genes 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.
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.