Fecal occult blood testing (FOBT) is used as a method to screen for colorectal cancer and to test for gastrointestinal bleeding in general. Traditionally, it has been performed using guaiac-based methods that detect the pseudoperoxidase activity of the hematin portion of hemoglobin. The guaiac-based methods are limited by a low sensitivity to anything but a clinically significant amount of gastrointestinal bleeding and the major drawback of a high incidence of false positive results especially when patients have not followed a restricted diet prior to testing. Immunochemical tests for FOBT have been studied as an alternative to guaiac-based tests. They are specific for human hemoglobin, avoid false positive results from peroxidase-containing foods in the diet, and are sensitive to relatively low concentrations of blood. One such immunologic FOBT, the FDA-approved Polymedco OC-Auto Micro 80, has been available through MuirLab since December 2005 and continues to meet clinical expectations.
The automated FOBT detects the presence of human hemoglobin by immunoassay using a photometric reading of the presence of an antibody-hemoglobin complex. There are many advantages of this over guaiac-based methods. Firstly, the photometric reading eliminates the subjectivity of interpreting a guaiac card test ("Is that blue?"). Secondly, as it is specific for human hemoglobin, there are essentially no false positives due to dietary peroxidase-containing foods. This eliminates the need for dietary restrictions for several days prior to testing, which potentially avoids a costly GI bleed workup. Finally, the immunologic FOBT is significantly more sensitive to the presence of hemoglobin than are guaiac-based methods, so it requires only ONE sample, rather than the three consecutive specimens recommended for the guaiac-based tests.
Regarding the sensitivity comment, it is known that in a normal state, a small amount of blood (i.e. 2-3 mL) is lost into the intestine each day. In a study by McRae and St. John, it was concluded that any amount over 300 µg hemoglobin/g stool was associated with gastrointestinal pathology. Thus it would be clinically useful to know the actual analytical performance of the various FOBTs that are commercially available, notwithstanding the claims in their package inserts. A well designed study was recently published by Tannous et al from the Massachusetts General Hospital/Harvard Medical School comparing a well known guaiac-based method to four different immunochemical methods including the one performed here at MuirLab. Using known concentrations of blood to spike a known weight of a normal occult blood negative fecal sample, they produced samples ranging from 1500 to 8 µg Hgb/g stool and tested each FOBT method in duplicate. The immunochemical method in use here at MuirLab showed an analytical sensitivity of 300 µg/g, whereas the well known guaiac method had an analytical sensitivity of 1500 µg/g. The other three immunochemical methods showed analytical sensitivities of 1500, 75 and <8 µg/g. This study reaffirms that the immunochemical method performed on site has the appropriate cutpoint to provide the necessary high sensitivity and low incidence of false positive results to optimize screening programs for colorectal cancer.
McRae F, St. John D. Relationship between patterns of bleeding and Hemoccult sensitivity in patients with colorectal cancers or adenomas. Gastroenterol 1988;94:A5.
Tannous B, et al. Comparison of conventional guaiac to four immunochemical methods for fecal occult blood testing: Implications for clinical practice in hospital and outpatient settings. Clinica Chimica Acta 2009;400:120-2.