Single-sample colorectal cancer test faulted by study
■ Colorectal cancer screening is effective, but not the single-stool blood test that is all too frequently performed in physicians' offices.
By Susan J. Landers — Posted Feb. 14, 2005
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Washington -- Sometimes the tried and true turns out to be neither. This is the case with physicians using a single-sample fecal occult blood test done in their office to screen for colorectal cancer.
The method is a poor screening tool, missing about 95% of precancerous advanced lesions, according to a study published in the Jan. 18 Annals of Internal Medicine. And a second study, also published in the Jan. 18 Annals, found that nearly one-third of primary care physicians surveyed who use the FOBT rely only on the single-sample office method to screen for colorectal cancer.
The American Gastroenterological Assn., for one, would like to change that.
Given that colorectal cancer is second only to lung cancer as the leading cause of cancer-related death in the United States, and that the transformation from benign colon polyps to invasive malignancy takes place over several years, early detection is key to reducing the high mortality rate.
Several recommended screening methods could help bring it down. Although the single-sample test is not among them, it still holds a place in the annual physical, especially for men older than age 50, said Emmet B. Keeffe, MD, AGA president.
Physicians often resort to the test because of poor compliance with the more accurate six-sample fecal occult blood test completed at home by patients. The thought is that one test is better than none.
However, the new study "really puts aside a standard procedure that we had learned was appropriate to do as part of a routine patient evaluation," said Dr. Keeffe.
The study that revealed the poor accuracy of the single-stool blood test was a prospective analysis of 3,121 patients, primarily men, age 50 to 75, at 13 Veterans Affairs medical centers. None of the participants had symptoms for colorectal cancer, and all agreed to have a colonoscopy whether or not they had a positive FOBT.
The researchers identified 284 people with colorectal cancer or large polyps likely to become cancer. Of these, only 4.9% had positive results from a single, office-based FOBT, while 23.9% had positive results from the six-sample test taken at home.
While obviously not perfect, the in-home FOBT has been shown to save lives, said Marion R. Nadel, PhD, an epidemiologist at the Centers for Disease Control and Prevention, and it is among screening tools recommended in the agency's guidelines.
Dr. Nadel conducted the study of physicians and their colorectal cancer screening practices. The study surveyed 1,147 primary care physicians and 11,365 people eligible for colorectal screening. Ninety percent of physicians surveyed said they used FOBT at least once per month, and one-third of these reported using only the single FOBT rather than the home test.
The patient survey results coincided with these findings.
In addition, nearly 30% of all physicians reported that they did not follow a positive FOBT with an immediate colonoscopy. National guidelines call for testing with colonoscopy as a follow-up to a positive FOBT.
"The message is that all adults 50 and older should be screened for colorectal cancer because we have strong evidence that screening can save lives. And if a patient and her physician choose FOBT for screening, then the home test should be used, and any positive results should be followed up with colonoscopy," said Dr. Nadel.
Dr. Keeffe agrees. "It is generally preferable to undergo colonoscopy, which allows complete evaluation of the colon and also, at the same time, biopsy or removal of most cancers or polyps that are identified," he said. "If detected early as precancerous polyps, the majority of colorectal cancers are preventable."
An editorial in the same Annals issue as the studies states that while the death rate from colorectal cancer has dropped from 29 per 100,000 in 1973 to 20 per 100,000 in 2001, the rate should be dropping more rapidly in the 10 years since effective screening became available.
Although people's reluctance to undergo colorectal cancer screening is part of the problem, the substitution of a poor screening test for effective tests may be another part of the problem, wrote Annals Editor Harold C. Sox, MD.
"Taken together, [the two studies] send a strong message to primary care physicians to reexamine their colorectal cancer screening practices. Perhaps we need to put the guaiac cards in a locked drawer labeled 'use only in case of emergency.' "