- For people with average risk of colon cancer, the American Cancer Society recommends screening for colon cancer between the ages of 45 and 75, while the U.S. Multi-Society Task Force (MSTF) on Colorectal Cancer and the U.S. Preventive Services Task Force (USPSTF) recommend screening starting at age 50.
- Every 3 years is typically recommended when you choose the stool DNA test for screening.
A stool sample is required. The sample may be collected in the privacy of your home. Typically, you will be supplied with a kit to collect an entire bowel movement and send it to a lab. Follow all the instructions that are provided with the kit to collect a stool sample that is not contaminated with urine. Follow the shipping instructions and timing because the samples must arrive at the testing laboratory within a specified time frame.
No preparation is necessary. However, you should collect a stool sample that is typical for you. For example, if you have diarrhea, you may need to wait to collect your sample until the diarrhea resolves.
The stool DNA test, also called a stool DNA-fecal immunochemical test, is a combination test that detects specific changes (mutations) in several genes associated with colon cancer within the cells shed in stool, as well as trace amounts of blood in stool. These genetic changes and small amounts of blood may be the first and sometimes the only signs of precancerous growths (polyps) or early colon cancer, making this stool-based test a useful screening tool. Detecting precancerous polyps and/or colon cancer early increases the chances that they can be successfully treated.
By combining two types of tests, the stool DNA test increases the chance of detecting polyps and colon cancer. The stool DNA tests for:
- DNA changes (mutations) associated with colon cancer—cancer is caused by changes in the DNA inside our cells. These changes can cause cells to grow and divide uncontrollably. The stool DNA test detects changes associated with colon cancer in the DNA of cells that are shed from the digestive tract into stool.
- Trace amounts of blood (hidden or "occult" blood)—although there are several possible causes of blood in the stool, one important cause is the presence of polyps or cancer in the digestive tract. Colon polyps are common as people age, but most polyps do not cause any health problems and are benign. However, a benign polyp can turn into a cancerous polyp and the cancer may even spread to other parts of the body (metastasize). Polyps are finger-like growths that protrude into the cavity (lumen) of the colon or the rectum. They can be fragile and bleed intermittently, such as when food waste brushes against them, leading to blood in the stool. This test uses a method called fecal immunochemical testing to detect trace amounts of blood (occult blood) in the stool. The fecal immunochemical test detects the globin (protein) part of hemoglobin in red blood cells. The test is specifically designed to detect globin from human blood and not from foods that you eat (e.g., meat). Therefore, dietary restrictions are not necessary before testing. Also, hemoglobin from bleeding in the upper digestive tract is broken down before it reaches the lower digestive tract and so upper GI bleeds are not usually detected by this test.
How is the test used?
This test is used to screen for colon polyps or early colon cancer by detecting digestive tract bleeding and certain gene changes (mutations) in stool, which may be indicators of colon cancer. It is one option for screening if you have an average risk of colon cancer. If cancer is detected early, treatment can begin immediately, improving the chance of a better outcome. Average risk includes people without a family history of colon cancer and those without certain genetic diseases.
This test is not recommended for people who have previously had adenomas of the colon (a specific kind of polyp) and those who have inflammatory bowel disease or increased risk of colon cancer. If you have an increased or high risk of colon cancer, a colonoscopy is usually recommended for screening because it is the most accurate and thorough screen available. See the article on Colon Cancer or the health screening articles for Adults and Adults age 50 and older for additional details.
Guidelines from the US Multi-Society Task Force on Colorectal Cancer recommend the stool DNA test as a tier 2 test. It is classified as tier 2 because it has some disadvantages compared to the recommended tier 1 tests, which are colonoscopy and annual fecal immunochemical test. (See the article on Colon Cancer for additional details on the different screening options.)
When is it ordered?
The American Cancer Society (ACS) and the US Multi-Society Task Force (MSTF) on Colorectal Cancer recommend testing every 3 years when you choose a stool DNA test as the method of screening for colon cancer. The U.S. Preventive Services Task Force (USPSTF) suggests every 1 to 3 years. This is because the stool DNA test is still relatively new, and the recommended screening frequency is still evolving.
The ACS advises that people of average risk begin screening at age 45. The MSTF and USPSTF advise that screening for colon cancer should begin at age 50.
What does the test result mean?
- A negative result on the DNA part of the test means the genetic changes associated with colon cancer have not been detected and it is less likely that you have colon cancer.
- If the test is positive, then it is more likely that cancer is present, but follow up is required. (See below.)
Stool immunochemical test
- A negative result means no hemoglobin was detected in the stool at the time of the test.
- A positive result indicates abnormal bleeding in the lower digestive tract. While this bleeding could be caused by colon cancer, other possible causes include ulcers, polyps or hemorrhoids.
Follow-up testing for positive results: The stool DNA test is a screening test. Positive results for either blood and/or DNA changes require follow-up testing. This usually involves direct imaging of the colon and rectum. A colonoscopy is typically recommended because a healthcare practitioner can examine the entire colon and remove any precancerous polyps and/or cancerous areas that are found.
Follow up for negative results: If the stool DNA test is negative, you should continue colon cancer screening at regular intervals. Guidelines recommend that the stool DNA test be repeated in about three years.
Do I need to stop taking any medications or are there any dietary restrictions for this test?
Can I just order the test on the internet and perform it at home?
You can talk to your healthcare practitioner, who can order the test for you, or you can request the test on the internet through a telemedicine provider. Once the test is ordered, the sample collection kit is mailed to your home. After you collect your sample at home, you will need to return the kit to a laboratory, where trained laboratory personnel perform the actual testing.
What other stool screening tests are available?
There are two other stool tests available for colon cancer screening. Both tests detect trace amounts of blood in the stool. Neither of these tests detects genetic changes in DNA. If one of these tests is used for colon cancer screening, then screening will need to be done annually.
- The first is the fecal immunochemical test that is not combined with a stool DNA test. It is preferred over the guaiac-based fecal occult blood test (see below) because it is sensitive enough that a single sample is considered enough for testing.
- The other is the guaiac-based FOBT (gFOBT, FOBT) test. Typically, gFOBT tests are performed on multiple stool samples collected on different days to better detect intermittent bleeding. You would be instructed to avoid certain medications and follow certain dietary restrictions for several days before collecting the stool samples.
Other than stool tests, are there other ways to screen for colon cancer?
Yes. There are imaging tests that may be used for the detection of precancerous polyps and colon cancer. You and your healthcare practitioner may select one of these procedures instead of screening with a stool DNA test or a fecal occult blood test:
- Colonoscopy is recommended by most health organizations as the preferred method of screening for colon cancer. It involves a thorough examination of the rectum and entire colon using a flexible tube. If polyps or potentially cancerous areas are found, they may be removed during the procedure and examined by a pathologist to see if cancer is present. Colonoscopy is recommended every 10 years for average-risk individuals. Colonoscopy, however, requires significant preparation, is invasive, and is much costlier than tests for occult blood in the stool. In individuals with abnormal results on the stool DNA test, colonoscopy is then used as a diagnostic test for colon cancer.
- Sigmoidoscopy is an examination of the rectum and lower colon with a lighted instrument. It also allows for the removal of any polyps. Sigmoidoscopy is recommended every 5 to 10 years.
- CT colonoscopy (virtual colonoscopy) is a less invasive procedure that uses computed tomography (a CT scan) to visualize the entire colon. The recommended screening interval is 5 years.
- Capsule colonoscopy is a procedure that uses a vitamin-sized capsule that is swallowed and contains a wireless camera that transmits pictures as it travels through the digestive tract. Not enough evidence is available on this test yet and it is not widely available. If chosen for screening, capsule colonoscopy should be done every 5 years.
Is there anything else I should know?
False-positive results: In the absence of polyps or colon cancer, a false-positive result for blood may be obtained if you have bleeding from other sources, such as hemorrhoids, ulcers or inflammatory bowel disease.