New DNA Stool Testing Spots More Colon Cancers

Study found it detected twice as many malignancies, serious precancerous polyps

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HealthDay Reporter

WEDNESDAY, Oct. 8, 2008 (HealthDay News) -- A new and improved DNA stool sample test to screen for colon cancer is twice as effective at catching cancer and serious precancerous polyps than either current blood stool sample tests or an older version of DNA testing, new research reveals.

"This is a very important finding in that if you don't detect the precancerous lesions, you don't actually prevent cancer," said Dr. David A. Ahlquist, a professor of medicine, and a gastroenterology consultant at the Mayo Clinic in Rochester, Minn.

"The problem increasingly sensitive DNA testing addresses is that blood as a marker for colon cancer has limitations," he explained. "Because most polyps basically don't bleed. In contrast, all precancerous polyps shed cells that contain abnormal DNA. So, a stool-based DNA test is a strategically more rational approach."

The finding was reported in the Oct. 7 issue of the Annals of Internal Medicine.

Ahlquist stressed that he and his team have "no commercial attachments whatsoever" to the new DNA stool test method, while noting that the study was supported by grants from the National Institutes of Health.

Though considered curable if detected early and before spreading, the authors point out that colorectal cancer ranks as the second most common cause of death among cancers in the United States, following lung cancer.

At the moment, several screening options exist, including radiographic imaging; invasive colonoscopy (involving insertion of a flexible tube into the colon); and the testing of stool samples for signs of blood and/or abnormal DNA shed by the colon. All these methods seek to uncover evidence of the precancerous growth of polyps on the lining of the colon, which develop anywhere from five to 10 years before cancer itself.

Yet despite the availability of such screening tools, the research team observed that more than half of American adults have never undergone colorectal cancer screening. This fact, they suggest, highlights the need for more user-friendly testing methods.

With this objective in mind, Ahlquist and his colleagues set out to assess the relative potential of the new DNA stool testing procedure among 3,800 healthy adults whom they determined had an average risk for developing colon cancer.

The goal was to stack up the new DNA fecal test -- referred to as "SDT-2"-- against both standard blood fecal testing and an older DNA fecal test, called "SDT-1". In this regard, the researchers described the methodology of the new SDT-2 test as potentially more "broadly informative" in terms of its ability to sift through and analyze DNA material.

All the participants collected three stool samples at home. The samples were then submitted for lab testing under the three different screening methods, although the newer SDT-2 test was not performed for all participants. All patients later underwent a colonoscopy to verify the accuracy of the three tests.

The results? The best blood testing was able to detect 21 percent of actual cancer cases and the most serious polyp growths. The older SDT-1 DNA test performed no better, uncovering 20 percent of cancer cases.

However, the newer SDT-2 DNA test was found to be much more effective -- detecting 40 percent of cancer cases and serious polyp growth.

Ahlquist and colleagues acknowledged that they have not yet evaluated the degree of false-positive results that the more sensitive SDT-2 DNA test may have triggered.

Nevertheless, they suggested that the newer and apparently more sensitive DNA stool screening method could represent a substantial advance in molecular screening technology. And although colonoscopy screening is still the more effective option to date, the team noted that such innovations could ultimately encourage more patients to undergo testing for colorectal cancer by providing easier, less invasive and reliable screening options.

Ahlquist said the newer DNA screening method is already approved and available for patient use.

"And subsequent generations will come out -- in a year or two -- with improved performance features," he noted, "which means that DNA test accuracy is only going to get better. So, this is a very promising alternative approach that is unfolding here, that we should pay attention to."

Dr. Donald Garrow, a clinical gastroenterology fellow at the Medical University of South Carolina in Charleston, said that DNA testing can be very helpful in combating colon cancer by giving doctors a way to motivate patients to take the next gold-standard screening step in the form of a colonoscopy.

"Fecal DNA tests are wonderful, in that they may spark more people to get a colonoscopy," he said. "They can never replace a colonoscopy. But if a trusted doctor comes to a patient and says that their DNA test has come back positive, that may be the form of encouragement the patient needs to go in for the colonoscopy."

Taking a fresh look at the benefits of more traditional testing, another article published in the same journal noted that the U.S. Preventive Services Task Force (USPSTF) is updating its prior 2000 screening recommendations with an eye to increasing life expectancy.

The USPSTF will now recommend that patients between the ages of 50 and 75 get screened for colorectal cancer once yearly with blood tests, once a decade with a colonoscopy, or once every five years with a combination of blood testing and a flexible sigmoidoscopy (a less invasive alternative to a colonoscopy). Routine tests will not be promoted, however, for those between the ages of 76 and 85.

The new testing guidelines appear to now track the American Cancer Society's (ACS) screening position. Last week, the ACS reiterated similar testing protocols for men and women over the age of 50 who have an average risk for developing colon cancer, while urging those with higher risk -- due to a personal and/or family history of the disease -- to begin screening at an even earlier age.

More information

For more about colon cancer screening, visit the Centers for Disease Control and Prevention.

SOURCES: David A. Ahlquist, M.D., professor, medicine, and consultant, gastroenterology, Mayo Clinic, Rochester, Minn.; Donald Garrow, M.D., clinical gastroenterology fellow, Medical University of South Carolina, Charleston; Oct. 7, 2008, Annals of Internal Medicine

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