Stool-Based Colon Cancer Tests Vary Widely in Accuracy

TUESDAY, Feb. 3 (HealthDay News) -- Newer versions of the stool-based colon cancer tests -- recommended annually for Americans over 50 -- vary widely in their ability to spot potential abnormalities, a German study finds.

Doctors at 20 gastroenterology practices in Germany used the tests for what is called fecal occult blood -- otherwise undetectable traces of blood that could come from a cancer or polyp in the intestine -- on more than 1,300 people and checked those results against findings from colonoscopy and an X-ray examination of the colon.

The study focused on the two types of tests for fecal occult blood, each of which uses a small stool sample placed on a card. The older test uses a chemical called guaiac, which can detect the "heme" part of the hemoglobin molecule in blood. However, the accuracy of the guaiac test can be compromised by the patient's diet -- for example, the recent consumption of red meat or even vitamin C supplements.

In 2000, a newer type of stool screen called the "immunochemical" test was developed. It doesn't spot blood directly, but rather picks up antibodies to the globin in the blood's hemoglobin molecule. This test is less easily "fooled" by diet, and soon after its introduction, the American Cancer Society recommended adding the immunochemical version to its screening guidelines, noting that the tests "are more patient-friendly and are likely to be equal or better in sensitivity and specificity" to the guaiac screen.

By now, a number of immunochemical tests have entered the marketplace. The researchers, from the German Cancer Research Center in Heidelberg, compared six tests against each other and against the older guaiac test. They reported their findings in the Feb. 3 issue of the Annals of Internal Medicine.

The team found a wide variance in diagnostic performance between the tests. The sensitivity for accurately spotting an adenoma (suspicious growth) ranged from 25 percent to 72 percent, while the tests' ability to accurately weed out "false positives" ranged from 70 percent to 97 percent.

One expert noted that stool-based tests were never the "gold standard" for colon cancer detection, so these results are not overly surprising. In that context, "both the guaiac-based test and the immunological tests seemed reasonably sensitive in detecting cancer," said Dr. Durado Brooks, director of colorectal cancer at the American Cancer Society. "But overall, they have relatively poor sensitivity for detecting [colon] polyps."

The implications of the test results are less clear for American physicians, Brooks added, partly because only one of the six immunological tests in the study is currently available in the United States, and partly because the researchers used an older, less popular version of the guaiac test.

In addition, the German researchers used only a single sample for the guaiac-based tests, while the American Cancer Society recommends testing three samples, Brooks said.

But another expert said these types of studies are needed.

"The important point of this paper is that you must test these methods in a screening population" before widespread use, said Ann Zauber, an associate attending biostatistician at Memorial Sloan-Kettering Cancer Center in New York City.

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