Colon Cancer Stool Screening Tests Vary in Accuracy

Writing in the February 3 Annals of Internal Medicine, German researchers noted that stool tests are an important component of colon cancer screening options, especially for patients who do not have access to or prefer not to have colonoscopy, considered the gold standard for screening and diagnosis. Stool screening tests are a relatively quick and non-invasive way to screen for early cancer, but they are less likely to identify precancerous polyps as do exams by colonoscopy. Stool screening tests are designed to detect the presence of blood. Positive stool tests are then followed by colonoscopy to determine the source of bleeding. Since many cancerous lesions tend to bleed only intermittently, tests for blood may miss lesions that colonoscopy would detect.

The most common type of screening for colon cancer is a guaiac-based fecal occult blood test (gFOBT). The test involves a small amount of stool smeared onto a piece of cardboard impregnated with a compound called guaiac. If a stool sample contains blood, guaiac turns blue when developer solution is added. One problem with the guaiac test is that any blood, including blood from red meats in the diet, will cause color change. In addition, many other dietary products and common medications can induce this same color change or can inhibit the color change, thus masking the presence of blood. For this reason, patients must follow dietary restrictions for several days prior to a guaiac test.

Fecal occult blood tests using antibodies (iFOBT) to detect hemoglobin from human blood are unaffected by diet or medicine. Patients do not need to restrict diet or stop medications prior to this type of FOBT.

Researchers compared the guaiac-based FOBT with six different antibody-based FOBTs in 1319 patients at average risk for colon cancer. After stool samples had been collected for testing, the patients had colonoscopy performed at one of 20 German gastroenterology practices. The researchers compared the results of the screening test with colonoscopy findings. Screening tests were interpreted without knowledge of colonoscopy results.

Thirty-one percent of patients had a type of colon polyp called an adenoma, detected by colonoscopy. Ten percent of these were considered advanced. All the immunochemical FOBTs were able to detect more of the patients with adenomatous polyps than the guaiac FOBT. However, the different immunochemical tests varied considerably in their sensitivity (being positive when polyps are present) and their specificity (being negative when polyps are not present). The two best performing antibody-based tests detected 25 to 27% of patients with polyps compared to only 9% by the guaiac-based test. All three tests had similar specificities of 93 – 97% (gave false positive test results for 3-7% of people with no polyps). The other immunochemical FOBTs did not have as high specificities.

Different tests also showed some differences in their ability to detect small lesions and lesions found in various locations in the colon.

The researchers noted, “Our results demonstrate that qualitative immunochemical FOBTs could be an option for future colon cancer screening, combining the advantages of immunochemical FOBT (improved detection of precancerous lesions) with the same or greater simplicity and practicality for mass screening as offered by the widely used guaiac-based FOBT.” However, they added that the exact role for immunochemical screening tests for detection of colon cancer remains to be determined.

Sources

MedlinePlus Medical Dictionary: Stool Guaiac Test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003393.htm. Updated March 8, 2009. Accessed February 26, 2009.

Sabrina Hundt, MSc et al. Comparative Evaluation of Immunochemical Fecal Occult Blood Tests for Colorectal Adenoma Detection. Annals of Internal Medicine 2009; 150: 162-169.

Summaries for Patients: Immunochemical Fecal Occult Blood Tests. Annals of Internal Medicine 2009; 150: 162-169.

Nicholas Bakalar. Quick Tests for Cancer of Colon Reviewed. The New York Times. Febuary 3, 2009. Available online at http://www.nytimes.com/2009/02/03/health/research/03colon.html?_r=1&emc=eta1 through http://www.nytimes.com. Accessed February 26, 2009.