In a study involving more than 13,000 patients, researchers at the Mayo Institute, based in Rochester, Minn., found that the popular Hemoccult test failed to detect more than 70 percent of colorectal cancers and more than 90 percent of polyps found in follow-up exams.
Researchers also looked at another test, HemoQuant, which is more sensitive in detecting fecal blood. But it, too, was found to be ineffective.
"Fecal blood is at best a flawed screening marker for colorectal neoplasia" -- abnormal development of cells that may or may not be cancerous, the authors said. "Those clinicians and patients who choose periodic fecal blood testing should be cognizant of its major limitations."
The study's principal investigator, Dr. David Ahlquist, a Mayo gastro-enterologist, said that the card tests have been used as a screening tool for more than 20 years and that it is common practice around the country for groups to pass them out free during community screening drives.
Fecal blood test kits, which are available at pharmacies without a prescription, cost $5 to $12.
Physicians routinely encouraged patients to use the screening tests because early studies showed they could detect 70 percent to 80 percent of colorectal cancers. But Dr. Ahlquist said the studies were flawed because they were based on groups of patients who had symptoms of the disease.
The fear is not that the tests will give a false positive results but that some patients who have early stages of colorectal cancer will be reassured falsely that they do not have the disease, Dr. Ahlquist said.
According to the study, which is published in today's Journal of the American Medical Association, the Hemoccult test accurately predicted cancer less than 10 percent of the time.
"The low test sensitivities observed were sobering," the authors said. "Clearly, better screening approaches are needed."
Colorectal cancer is second to lung cancer in the number of people it kills. Last year, it killed more than 60,000 Americans.
But colorectal cancer, while deadly, is one of the more curable cancers if detected early.
While there are numerous methods to screen for colorectal cancer, most general practitioners in the United States use periodic fecal blood testing for preventive health care in adults, the study said.
Although fecal blood testing has been endorsed by the American Cancer Society, the National Cancer Institute and the American College of Physicians, its effectiveness in people who do not have symptoms has never been proven, the study said.
Until more accurate screening methods are developed, Dr. Ahlquist said, doctors and patients must rely on more expensive, less convenient tests, such as colon X-rays and colonoscopies.