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Fecal Occult Blood Immunochemical

Colorectal cancer is the third most common form of cancer and the third leading cause of cancer death among both men and women in the United States. The most significant risk factor is age: More than 90% of colorectal cancers are diagnosed in people older than 50. Annual screening with a fecal occult blood test (FOBT) can decrease colorectal cancer mortality by up to 33%. Current guidelines recommend an annual FOBT for individuals at average risk beginning at age 50. Individuals considered to be at high risk should begin screening at age 40.

The original FOBT were guaiac based and detected the peroxidase activity of heme. For more information about the performance of this test, please refer to Fecal Occult Blood Test. The major drawback of this test was that it was not specific for human hemoglobin and detected heme in red meat and peroxidase from fruits and vegetables. To avoid false positive results, strict dietary and medication restrictions were necessary, which decreased patient compliance. Immunochemical FOBT (iFOBT) or fecal immunochemical test (FIT) were developed to specifically detect intact human hemoglobin. This test uses monoclonal antibodies directed against the globin moiety of human hemoglobin. These antibodies do not react with hemoglobin from nonhuman dietary sources. They also do not detect partly digested human hemoglobin from the respiratory or upper gastrointestinal tract. Thus, iFOBT is more specific than gFOBT

Hemoccult ICT (Beckman Coulter) uses the principle of immunochromatography to detect human hemoglobin from blood in fecal samples. The test kit includes a collection card and test device. Feces from two different areas of a stool is smeared onto the collection card. The card is transported to the testing site where the dried sample is transferred to the immunochromatograhy test device.  Buffer is added to the dried stool to extract any hemoglobin that may be present. Solubilized sample flows down the test strip and rehydrates the colloidal gold anti-human hemoglobin antibody conjugate that is dried onto the test strip. If hemoglobin is present in the sample, it binds to the colloidal gold anti-human hemoglobin antibody.  This immune complex is captured on the test strip in a zone containing anti-human hemoglobin antibodies to form a visible colored line. If human hemoglobin is absent in the stool sample, no line is visible. Unbound conjugate continues to migrate down the test strip and binds to the Control Line which contains conjugate-specific antibodies.

When testing an asymptomatic average risk population, Hemoccult ICT’s positive rate for multi-day screening was 1.8% compared to 2.9% for guiac based Hemoccult. The lower positive rate of Hemoccult ICT is due to its higher specificity for human hemoglobin.

Sensitivity and specificity of iFOBT were 79% and 94%, respectively, which is similar to gFOBT. There does not appear to be a statistically significant difference between FIT and gFOBT for detection of advanced adenomas.

Generally, iFOBT testing is more sensitive for cancers than for benign adenomas. Some studies have suggested that sensitivity for cancer is higher with the use of low-dose aspirin while specificity was only slightly lower.

Among those patients with a positive iFOBT and a negative colonoscopy, the most common risk factors associated with a false-positive fecal test result were use of anti platelet drugs and a low hemoglobin concentration. False-negatives can occur because of uneven distribution of blood in the feces or intermittent bleeding.

Clinical studies have not found consistent differences in test performance between various immunochemical FOBTs. iFOBT kits cost approximately five times more than guaiac FOBT.

 iFOBT require samples from one, two or three stools, depending on the manufacturer of the test.

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