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Focal occult blood test (FOBT)

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Focal occult blood test (FOBT)

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  • May 20, 2009
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Uncle Buck, a 50 year old health conscious guy, goes in for his annual checkup including glucose, TSH, total cholesterol, CBC, and a fecal occult blood test. The results were all normal, except for a positive FOBT. The risk for colon cancer polyps begins at age 50, and the diagnostic yield increases with age. A positive FOBT indicates blood in the stool, however, the traditional guaiac reaction used in the lab for Uncle Bucks case has some interferences associated with it. A colonoscopy for Uncle Buck was the follow-up test to the FOBT to detect polyps in the intestine. However, Uncle Bucks colonoscopy results were negative, along with detecting still another positive FOBT before he leaves the physicians office that same day. Colorectal lesions are usually revealed in a colonoscopy; however, miss rates of adenomas do occur if the screening is done too rapidly, or if screening is done consecutively. The early stages of colon cancer can be also be missed on a colonoscopy if the disease has not progressed to a detectable level. However, Uncle Bucks case is most likely not colon cancer, but rather a discrepancy with the screening methods.

The FOBT has traditionally been performed using guaiac-based methods that detect the peroxidase activity of heme in blood and other peroxidases (3). Guaiac gum is saturated on a piece of filter paper. If heme oxidase is present, after hydrogen peroxide has been added to oxidize peroxidases present in heme, there is a color change observed. If a blue color does not emerge within 1 minute after hydrogen peroxide is added, the test is negative (3). Technical expertise is needed to carry out this complex test, so that an accurate result is generated. Only experienced personnel should carry out G-FOBT. Guaiac-based FOBT has a lower sensitivity and thus limited diagnostic performance for detecting colon cancer. The G-FOBT is used only for blood in stool, not in urine (3). Thus, several interferences could result from using this method...