The main use for the fecal occult blood test (FOBT) or the fecal immunochemical test (FIT) is as a screen for early colon cancer. Most cases of colon cancer begin with the development of benign intestinal polyps. Benign polyps are relatively common in people over the age of 50. Most are harmless, but some can become cancerous.
Blood in the stool may be the only symptom of early cancer; thus, if detected early, treatment can begin immediately, improving the chance of a cure.
Methods of testing for fecal occult blood include:
The guaiac smear method (gFOBT) — uses a chemical indicator that shows a color change in the presence of blood
An over-the-counter (OTC) flushable reagent pad/tissue method also produces a color change in the presence of blood.
Immunochemical method (iFOBT or FIT) — uses antibodies directed against human hemoglobin to detect blood in the stool
A secondary use of FOBT is to determine the cause of anemia, such as blood loss from a bleeding ulcer. If someone has signs and symptoms of anemia, such as fatigue, a low hemoglobin and hematocrit, and/or unusually dark stools, a health practitioner may order the FOBT.
The American Cancer Society (ACS) and other major healthcare organizations recommend yearly testing when a person chooses fecal occult blood testing as the method of screening for colon cancer. The ACS and others advise that screening for colon cancer begin at age 50 for the general population, but it may begin at an earlier age when a person has a family medical history of colon cancer.
A health practitioner may sometimes order an FOBT when someone has unexplained anemia that might be caused by bleeding in the digestive tract.
For the fecal immunochemical test (FIT), a positive result indicates abnormal bleeding in the lower digestive tract. Since this test detects only human hemoglobin, other sources of blood, such as from the diet, do not cause a positive result. Moreover, hemoglobin from bleeding in the upper digestive tract is broken down before it reaches the lower digestive tract and is not detected by the FIT. Thus, the FIT is a more specific test than gFOBT.
A positive result from either the guaiac-based FOBT or immunochemical FIT requires follow-up testing. This usually involves direct imaging of the colon and rectum (sigmoidoscopy or colonoscopy).
Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples. Taking three different samples on three different days increases the chances of detecting bleeding that is intermittent.
This article was last reviewed on January 28, 2015. | This article was last modified on January 28, 2015.
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