A stool DNA test is a new approach for colon cancer screening. A stool DNA test is designed to identify recognizable DNA changes (DNA markers) in cells that are continually shed from the lining of the colon through stool. These markers are associated with the surfaces of the cells of precancerous polyps and cancerous tumors. Because the lining of the colon is always shedding cells, including cells from the surface of polyps and tumors, these markers are available in the stool. A stool DNA test can identify several of these markers, indicating the presence of precancerous polyps or colon cancer.
Because DNA changes may differ between colon cancers, stool DNA tests typically target multiple markers to achieve high detection rates. Also, because DNA markers may be present in only trace quantities in stool, very sensitive laboratory methods are required. Stool DNA testing has been shown to be more effective than fecal occult blood tests at detecting colon cancer and precancerous polyps. The new stool DNA tests demonstrate high detection rates of early-stage colon cancer. Unlike other noninvasive tests, the new stool DNA tests also can detect precancerous polyps.
One stool DNA test is currently available, but it has not been approved by the Food and Drug Administration (FDA). Next-generation stool DNA tests that have much improved detection rates are undergoing final clinical validation in a multicenter study by the FDA.
The method of screening for colon cancer using stool DNA testing has been endorsed by the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer and the American College of Radiology, but not by the U.S. Preventive Services Task Force.
Why it's done
The stool DNA test is a screening test for colon cancer and precancerous polyps.
How you prepare
Stool DNA testing requires little preparation. You can eat and drink normally before the test and continue your current medications. There's also no need to clean out (empty) your colon ahead of time.
What you can expect
The stool DNA test that's currently available isn't approved by the FDA, but it may be prescribed by a doctor under special rules. You'll receive the stool DNA test kit for collecting and submitting your stool sample at your doctor's office or by mail if you order it online. The kit includes a container that attaches to the toilet. After you collect the stool sample, you either return the kit to your doctor or mail it to the lab. The test requires only one sample.
Various methods to improve the ease of stool collection and sampling are under evaluation. The stool DNA test may be repeated every two to five years, but additional research must be done to determine the optimal interval for testing. In addition, Medicare currently doesn't cover the test, but some private insurers do.
Your doctor will review the results of your stool DNA test and then share the results with you.
- Negative result. A test is considered negative if DNA markers common to colon cancer or precancerous polyps are not found in your stool.
- Positive result. A test is considered positive if DNA markers common to colon cancer or precancerous polyps are found in your stool sample. Additional evaluation — usually colonoscopy — would be recommended to determine whether you have cancerous or precancerous changes in your colon or other parts of your digestive system.
- False-negative result. A false-negative result — a negative test result when cancer is present — may occur if colon cancer or polyps do not harbor DNA markers targeted by the stool DNA test, or if markers are present in extremely low amounts. While next-generation stool DNA test methods appear to be capable of detecting most colon cancers and precancerous polyps, further study is needed to determine what the rate of false-negative results will be.
- False-positive result. A false-positive result — a positive test for cancer when no cancer is present — may occur in about 5 to 10 percent of people screened. False-positives could be due to the presence of tumors above the colon or to a problem with the test. When the stool DNA test result is positive but a follow-up colonoscopy is normal, your doctor may recommend further observation with another stool DNA test, evaluation of your upper gastrointestinal tract, a repeat colonoscopy or a combination of these. Research is being done to clarify the best way to manage false-positive results.