Improved Stool Screening Test Finds DNA Changes

Too many people avoid colonoscopy.  Too invasive, they say.  Too scary, too risky.

There may be an answer for them in an improved stool test that looks for DNA that is changed in both colorectal cancer and some precancerous polyps.

Of course, if the stool test identifies possible polyps or cancer, a colonoscopy is critical to evaluate the findings and remove polyps.

The test that looks for methylated DNA in human feces, found 85 percent of cancers and 64 percent of large adenomas.  There were few false positives.  Only one in ten follow-up colonoscopies didn’t confirm cancer or adenomas.

Reported at the AACR Colorectal Cancer: Biology to Therapy conference in Philadelphia in October, the screening test was validated in 1,100 patients.

DNA methylation is a chemical change that plays a crucial role in the development of almost all cancers, including colon and rectal cancer.   Methylation can change the way cells grow out of control and develop into cancer. Cells with the altered DNA are shed constantly from the colon wall and from polyps into stool and passed during bowel movements.


The test was most sensitive for early cancers, which are potentially curable, finding 87 percent of stages I through III.  Strangely, it only found 69 percent of stage IV colorectal cancers,a loss DNA methylation which the scientists can’t explain.

As adenomas grow, the test become more sensitive.  It only finds 31 percent of polyps under 1 centimeter, but 64 percent of those over a centimeter, 79 percent of polyps bigger than 2 cm, and 91 percent of adenomas larger than 3 cm.

Sensile or flat polyps were also detected.

It was equally effective in finding both polyps and cancers in the upper and lower parts of the colon — a problem for colonoscopy, which may miss polyps in the upper or proximal colon.  Age or gender didn’t make a difference.


Dr. David Ahlquist pointed out the value of the stool DNA screening test.

There is definitely an incentive and legitimate justification to be designing a screening approach that is user friendly, affordable and has the ability to detect pre-cancers. The noninvasive stool DNA test we have developed is simple for patients, involves no diet or medication restriction, no unpleasant bowel preparation, and no lost work time, as it can be done from home. Positive tests results would be followed up with colonoscopy.

How About Now?

The new test, being developed by Exact Sciences, is simpler than earlier stool DNA testing both for patient and for the testing system.  The faster, simpler test may be less expensive when it is on the market.

But, for now, the test remains experimental.  A large clinical trial, involving at least 8,000 people will begin early next year, which may lead to FDA approval for screening use in the United States.

During the discussion at AACR conference, Ahlquist was asked what doctors should do if patients read about the new screening test and ask for it.  He replied that it is not yet ready for the general public and that patients should be encouraged to have a colonoscopy now.

Learn More

A video from Exact Sciences provides additional information about how the stool DNA test works.

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