A fairly simple and inexpensive fecal occult blood test (FOBT) that detects blood cells in a person's stool sample has been found to be an effective way to screen for colon cancer, according to a recent study published August 2, 2011 in the Canadian Medical Association Journal (DOI:10.1503/cmaj.101248).
The results are a strong support for the use of the iFOBT as an effective screening test. Of the 2796 asymptomatic participants from Taiwan who took part in the study, 397 had a positive fecal test result. The authors demonstrate that the iFOBT was specific for predicting lesions in the lower GI tract but did not adequately predict lesions in the upper GI tract.
The study aimed to address whether asymptomatic patients with a positive FOBT should undergo evaluation of the upper gastrointestinal (GI) tract for cancer using a newer version of this test called the immunochemical FOBT (iFOBT). According to the authors, there has been little previous evidence on the specificity of this test for colon cancer.
Compared to the older guaiac-based FOBT which may have confounded results by detecting bleeding from other upper GI sources such as stomach ulcers, the new test can more accurately detect bleeding specifically from the lower GI tract. The iFOBT allows for better detection of lower GI bleeding as it is able to identify a specific protein that is normally digested by enzymes within the upper GI tract.
The study showed a 24.3% sensitivity for detecting neoplasms in the colon and a specificity of 89% for predicting colorectal cancer, adenoma, or any other important lesion in the lower GI tract. The accuracy of the test was 73.4%.
Lesions in the lower GI tract were higher among those with a positive FOBT test (P < .001) while the prevalence of lesions in the upper GI tract did not differ between those with a positive or negative FOBT test (P = .12). Of the participants diagnosed with colon cancer, 27 of the 28 (96.4%) had a positive fecal test. The risk factors associated with a false-positive fecal test were the use of antiplatelet agents and a low hemoglobin concentration. Among the participants, the three that were found to have cancers of the stomach or esophagus did not have a positive iFOBT test.
Current Practice
Experts generally recommended colon cancer screening tests starting at age 50 for those of average risk for disease development. Screening test options include an annual stool test, an invasive colonoscopy every ten years or a flexible sigmoidoscopy every five years. Any of these screening tests are supported by the US Preventive Services Task Force. Following a positive stool test, a colonoscopy is recommended to elucidate the source of the blood in the stool. Blood in the stool is a potentially sign of precancerous polyps or colon cancer. However, anywhere from 13% to 42% of those with a positive FOBT test end up with a negative colonoscopy.
Currently, the lifetime risk of colon cancer in the United States is 1 in 19 and more than 50,000 Americans died from the disease in 2010. Cancers of the upper digestive tract are more rare, with a 1 in 200 chance for esophageal cancer and a 1 in 114 chance for stomach cancer.
Currently, a pan-detecting assay that is also based on stool samples is being developed, allowing for multiple screenings of digestive tract cancers. Researchers are currently looking into whether combining the guaiac test with the newer iFOBT coud be useful for detecting both upper and lower GI cancers. An added third stool test that spots ulcer-causing bacteria H. pylori, is also being studied. H. pylori infections are associated with increased risks of stomach and throat cancers.
The major benefit of these tests is a less invasive screening tool that is also much cheaper. The iFOBT is about $30 compared to a $3000 colonoscopy.
