Fecal Occult Blood Test is a test carried out to detect the presence of blood in stool that cannot be seen macroscopically. This test is very important in the diagnosis of colorectal cancer.There are various ways of carrying out this test. The first method of conducting this test is through immunochemical testing also known as iFOB. There are two types of iFOB; Qualitative and quantitative iFOB. Quantitative iFOB measure the concentration of hemoglobin in the stool through an automated analyzer. Since it is able to quantify the amount of hemoglobin in stool, it can predict the amount of bleeding that is being experienced within the GIT as this has various clinical implications. Qualitative iFOB just detect whether there is hemoglobin in the stool or not without quantifying. These are mainly used for rapid diagnosis (Kościelniak-Merak, Radosavljević, Zając, & Tomasik, 2018). The advantage of iFOB is that there is no interference from heme derived from animal meat or chlorophyll plant diet making it more accurate in diagnosing bleeding from the upper GIT. It also offers the patients a convenience of not having to refrain from eating certain types of foods before taking the test. This method makes use of antibodies to detect globin in faeces. This is the method that is recommended by gastroenterologists for the detection of colorectal cancer since it is more efficient in terms of cost and health outcomes.
Another method of conducting the test is by doing the guaiac FOB. This method works more like the famous litmus paper whereby color forms the basis of diagnosis. The test involves smearing of faeces onto a piece of paper. Hydrogen peroxide is then added. If the test is positive, there will be a color change into guaic blue. The basis of this test is that hydrogen peroxide causes oxidation of guaic acid a process that is mediated by the heme which is a component of hemoglobin (Kościelniak-Merak, Radosavljević, Zając, & Tomasik, 2018). The problem with this test is that it can result in false positive results which might lead to misdiagnosis. In order to ensure accuracy of the test, the patient should be advised not to consume foods that might lead to false positives such as red meat and some vegetables 2 days prior to carrying out the test. Red meat might causes a false positive result because of the heme derived from the animal meat while vegetables cause a false positive result because chlorophyll in these vegetables possess pseudo-perioxidase activity thus mimicking the role heme-perioxidase.
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Carrying out the TMB testing which also utilizes the principle of litmus paper in diagnosis is another method of conducting the test. The heme present in stool, which is usually derived from blood in the GI tract, catalyzes the oxidation of the indicator. Usually this reaction occurs in an aqueous environment hence the need to dip the indicator in the toilet bowl. The test is considered to be positive if the indicator changes color to green/blue (Kościelniak-Merak, Radosavljević, Zając, & Tomasik, 2018). This test can also have false positive results because heme from animal meat and chrophyll from some vegetables can also cause oxidation thus giving false positive results. Patients should be advised not to take these foods at least 2 days prior to taking the test. The test is preferred because of its simplicity. However, it is subject to bias in the sense that color change observation is subjective. This test involves dipping a paper soaked in TMB toilet bowl soon after defecation. IN The test can also be conducted through DNA testing whereby mucosa cells found in faeces are assessed for any changes in DNA that is associated with malignancies.
The dependability of a test is diagnosing a particular condition is usually described in terms of specificity, sensitivity, predictive value, reliability, validity and reliability. FOBT has content validity in diagnosis of colorectal cancer in that it tests the presence of products of breakdown of red blood cells within the GI tract. Since colorectal cancer presents with GI bleeding, the test is valid in this sense. The test can also be said to be reliable in the diagnosis of Colorectal cancer in that the results obtained after testing can be replicated if the test is repeated some other time if at all the factors that might lead to false positive results are controlled. Sensitivity of test can be described as the ability of a test to identify with precision those with the condition being tested. FOBT can be said to be said to be highly sensitive for diagnosis of colorectal cancer although the sensitivity is not 100% since bleeding within the GI tract can be caused by other conditions like PUD disease, polyps, amoebiasis, among other conditions. iFOBT has a sensitivity of 50% in diagnosing Colorectal cancer while gFOB test have a sensitivity of 30% in diagnosing colorectal cancer (Kościelniak-Merak, Radosavljević, Zając, & Tomasik, 2018). Specificity of a test can be described as the ability of a test to correctly identify those without the condition under investigation. FOBT can be said to be highly specific in the diagnosis of colorectal cancer. Most cases of colorectal cancers present with GI bleeding and therefore absence of blood in stool is highly suggestive of not having a malignancy. iFOB is more specific than gFOB test in the diagnosis of Colorectal cancer since it does not suffer from interference from external factors. The specificity of iFOB in testing for colorectal cancer is considered to be 96.9% while that of gFOB test stands at 90%. The test might not rule out early stage disease hence it is not 100% specific. The predictive value of FOBT is generally high.
References
Kościelniak-Merak, B., Radosavljević, B., Zając, A., & Tomasik, P. J. (2018). Faecal Occult Blood Point-of-Care Tests. Journal of Gastrointestinal Cancer , 49 (4), 402–405. doi: 10.1007/s12029-018-0169-1
