Stool DNA

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.)

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.

Stool DNA

• 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.

No, there is no need to stop taking your medications and you can continue eating the same foods you normally do.

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.

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.

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.

There are a small number of false positives and false negatives associated with the stool DNA test.

False-negative results: Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples and may or may not be present in a specific sample.

The stool DNA test detects changes in specific genes. It is possible to have colon cancer that does not have changes in these genes, so cancer would not be detected with the test.

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.