Colon Cancer Screening Test Called Inaccurate

Common Test Performed in Doctor's Office Often Misses Cancer, Researchers Say

From the WebMD Archives

Jan. 18, 2005 -- A test commonly used by doctors to screen for colorectal cancer may miss up to 95% of potential cancers, according to two new studies.

In order to test for signs of colon cancer, doctors often check a sample of stool for blood. But researchers found that a single-sample fecal occult blood test (FOBT)fecal occult blood test (FOBT) performed in a doctor's office found only 4.9% of colon cancers or precancerous growths. However, nearly one-third of primary care doctors say they use the test in screening their patients for colon cancer.

Colon cancerColon cancer is the second leading cause of cancer deaths in the U.S.

Based on these results, researchers say use of this inaccurate test may help explain why the decline in colorectal cancer deaths has been surprisingly slow over the last decade.

Current colon cancer screening guidelines call for adults over 50 to take a six-sample FOBT test at home and return the stool samples to their doctors for evaluation. Presence of blood in the stool sample may indicate the presence of a growth or tumor in the colon, and further screening is usually recommended with more invasive testing, such as colonoscopy.colonoscopy.


But researchers say many people do not complete the home FOBT tests, and doctors sometimes perform a single-sample digital FOBT test in the office.

Researchers say this substitution may cause doctors to miss cancers that would have been picked up by the six-sample fecal occult blood test or other, more accurate colon cancer screening methods.

The results of the studies appear in the Jan. 17 issue of the Archives of Internal Medicine.

In-Office Test Overused, Inaccurate

In the first study, researchers compared the accuracy of fecal occult blood testing using a single sample obtained while a doctor performs a rectal examinationrectal examination versus the recommended six-sample FOBT.

More than 2,600 adults over age 50 had both tests along with a colonoscopy, which is considered the most accurate colon cancer screening test.

Colonoscopy screening showed that 284 had cancer or large polyps that were likely to become cancer. But less than 5% of these cases were picked up by the single-sample fecal occult blood test. In other words, the office-based FOBT missed 95% of precancerous or cancerous growths.


However, the six-sample test fared much better and detected nearly 24% of these cancers and precancerous growths.

In the second study, researchers surveyed doctors and patients to see if they followed recommendations for colon cancer screening using FOBT.

More than 90% of the 1,147 primary care providers surveyed said they used FOBT at least once a month. One-third of those doctors said they used the test only in their office rather than giving their patients the six-sample at-home test.

Nearly a third of the doctors also said that if their patient had a positive result on an in-office FOBT, they would follow up by repeating the same test rather than referring the patient for colonoscopy, as recommended.

About one in four of the more than 11,000 adults over 50 reported having a fecal occult blood test, and about a third said they had the test in the doctor's office only. Less than half of those who had a positive result on an in-office FOBT said they had been for further testing with a colonoscopy.


Experts call the results of the studies a "shocker."

In an editorial that accompanies the study, Harold C. Sox, MD, editor of the journal, says, "These two studies discredit the office-based, single-sample screening test for occult blood while simultaneously showing that it is a common practice."

"Taken together, they send a strong message to primary care physicians to re-examine their colorectal cancer screening practices."

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SOURCES: Collins, J. Nadel, M. Annals of Internal Medicine, Jan. 18, 2005; vol 142: pp 81-81, 86-94.
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