Don’t assume a normal annual test for fecal occult blood (FOBT) means you can postpone your colonoscopy. According to Frank Herlong, M.D., Associate Professor of Medicine, Gastroenterology Division at Johns Hopkins and Health After 50 Board Member, FOBTs are no longer relied upon for colon cancer screening -- though some doctors may use them to test for gastrointestinal bleeding from causes other than cancer.
Before colonoscopies became widely available and covered by insurance, annual FOBTs were an affordable and easy-to-use tool for colon cancer screening. But FOBTs have always been notoriously unreliable. Most employ a chemical called guaiac that changes color if blood is present in the stool sample. Guaiac also reacts with certain fruits and vegetables and with blood from red meat, which increases the likelihood of “false positive” results. All positive results must be confirmed by colonoscopy.
Immunochemical FOBTs, or IFOBTs, test for a protein specific to human blood. They are widely available and more accurate than guaiac based tests. But no matter which FOBT you use, a negative result does not necessarily mean that you are safe. Case in point: A recent study published in The New England Journal of Medicine found that an expensive DNA-based FOBT was capable of detecting 26% more invasive cancers and potentially precancerous growths than a common guaiac-based FOBT. But even this vastly superior DNA-based test identified only 51% of all invasive cancers detected by colonoscopy.
The most important thing to keep in mind is that not all precancerous growths or colon cancers cause bleeding; by the time they do, the cancer may be advanced. In contrast to all types of FOBTs, a colonoscopy detects -- and removes -- growths whether or not they cause bleeding. Thus, colonoscopy is your best protection against colon cancer.
