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NEWS & VIEWS

IN-OFFICE FOBT INACCURATE, BUT WIDELY USED

In a survey of 1,147 physicians analyzed by researchers from the ACS, the US Centers for Disease Control and Prevention (CDC), the National Institutes of Health, and the University of North Carolina at Chapel Hill, nearly 33% said they use this method exclusively to screen asymptomatic, average risk patients for colorectal cancer. Yet the test detects fewer than 5% of colorectal cancers, according to researchers with the Department of Veterans Affairs.

Those findings point to a "disturbing pattern of low-quality fecal occult blood testing and follow up, indicating that millions of adults are undergoing testing that, for the most part, is entirely worthless," said Robert Smith, PhD, director of cancer screening for the ACS and coauthor of the report on doctors’ use of in-office versus take-home tests.

The ACS recommends colon cancer screening for all adults at average risk starting at age 50. A yearly fecal occult blood test (FOBT) is one of the acceptable screening methods—but only when the multisample take-home kit is used. FOBT performed on a single stool sample taken during a rectal exam is not considered an adequate, or acceptable, substitute.

The Veterans Affairs study shows clearly why in-office FOBT poses a problem. This study compared the two methods of FOBT—a single sample collected by digital rectal examination (DRE) versus six samples collected at home (two samples from each of three stools collected on consecutive days)—in 2,665 patients who also had colonoscopies. The take-home FOBT kit found 23.9% of the advanced neoplasms (defined as adenomas with diameter 10 mm, adenomas with a villous component of 25%, adenomas with high grade dysplasia, or cases of intramucosal or invasive carcinoma) found by colonoscopy, while the in-office FOBT found just 4.9%. Among patients with invasive cancer, the take-home test was positive in 29.8% and the digital FOBT was positive in only 9.5%.

Expressing these results as likelihood ratios makes the deficiency of the in-office test even more apparent. The positive likelihood ratio (true positives/false positives) for advanced neoplasms was 1.68 (95% CI, 0.96 to 2.94). The corresponding negative likelihood ratio (false negatives/true negatives) was 0.95 (95% CI, 0.95 to 1.01). In other words, a negative result was as likely to be a false negative as a true negative. A likelihood ratio of 1 means, by definition, that the test result is completely useless.

"Normal test results from [in-office FOBT] are no assurance whatsoever that colorectal cancer is not present," Smith said. "Health care providers should abandon the practice of in-office FOBT for colorectal cancer screening immediately."

Using the wrong screening test isn’t the only problem, though. Smith’s study also found that many doctors aren’t doing proper follow up when an FOBT is positive.

The ACS and GI Consortium guidelines specify that patients with positive FOBT results should be sent for follow-up colonoscopy. Yet of 1,120 doctors who reported their follow-up procedures in the study, 29.7% said they give patients another FOBT if the first test turns out positive. Of the 925 doctors who reported a follow-up procedure other than FOBT, 52.8% said they send patients for colonoscopy and 22.5% said they send patients for sigmoidoscopy. An additional 15.7% said they give patients a double-contrast barium enema, sometimes with along with sigmoidoscopy or colonoscopy.

The findings suggest a need for greater awareness of screening guidelines, Smith and his coauthors wrote.

"Although the 24% sensitivity of the take-home FOBT for detecting advanced neoplasms initially sounds unimpressive, it’s important to remember that the cumulative sensitivity of annual testing is much higher. The evidence from prospective randomized trials has shown that colorectal cancer mortality can be reduced by 30% or more from proper application of [FOBT], which means annual at-home collection of six samples from three consecutive bowel movements," Smith said. "The key message to patients and clinicians from these two studies is that the convenience of office testing is no substitute for proper use of the test."

FOBT for colon cancer screening should be done with the multi-sample take-home kit, not a single stool sample collected in an office visit.

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