Fecal Occult Blood Test (FOBT)



Detection of occult bleeding - FOB (Fecal Occult Blood) must be specified in terms of clinical indication. Setting the cut-off value of the concentration of hemoglobin in stool is then a crucial question of the laboratory test. Physiologically, 0.5-2.5 ml of blood is lost daily in the stool. If we recalculate this amount with hemoglobine concentration in blood (120-150 mg/ml) and the amount of stool in 24 hours (300-450 g), then we can consider the values of 0.3-1.3 mg hemoglobin per 1 g of stool as physiological range. It means that this concentration would not be detected by FOBT screening as a positive finding.

There are two different approaches to detecting blood in the stool
 * heme detection (pseudoperoxidase, chemical reaction)
 * immunochemical detection of protein - globine.

Screening
For the screening procedure, the recommended test is Guemocakult Test - Haemoccult (gFOBT). Screening, as an initial method of screening programs for colorectal tumors in asymptomatic individuals, should be performed by a test that meets the established criteria. Screening programs are based on repeated determinations at regular intervals of one to two years. In case of a positive test result, a targeted gastroenterological (endoscopic) examination must follow to clarify the cause of the test positivity. For these reasons, immunochemical tests that have significantly higher sensitivity and provide 4-7% false positivity cannot be used for screening. Haemoccult test showed a positivity in 2.8% of cases in more than 95,000 asymptomatic patients in the study performed by ÚKBLD 1. LF UK and VFN, false positivity tested in comparison with immunochemical tests was zero. Controlled FOB screening significantly reduces the incidence of colorectal cancer.

Imunochemické testy (iFOBT)


Immunochemical tests (iFOBT) are suitable for the detection/exclusion of bleeding in symptomatic patients, where the occult bleeding test is one of a number of examination procedures. Sensitivity and positive detection are also significantly affected by the different degradation of both hemoglobin components with respect to the proximodistal gradient in the digestive tract. Globine is degraded much faster, and the positivity of immunochemical tests almost eliminates the detection of bleeding in the upper gastrointestinal tract. Heme degradation is significantly slower, so chemically oriented FOBT tests, such as haemoccult, can be also sensitive to bleeding from higher areas of the GIT. Hemagglutination, latex immunoprecipitation, radial immunodiffusion and immunoaffinity chromatography tests are based on the immunochemical principle. Protein detection (human hemoglobine) of monoclonal antibody eliminates the possibility of other sources of hemoglobine (food). No interference from chemicals is here, so no special diet is required. The sensitivity of immunochemical tests is significantly higher - depending on the technique and less than 0.1 mg hemoglobin/g stool. Quantitative hemoglobin determination - qiFOBT - correlates with bleeding rates of precancerous lesions (adenoma) and colon tumors. Studies in recent years have tested several immunochemical analyzers for the quantitative determination of blood in the stool, most of which are Japanese-made. ROC curves show 95.3% specificity for advanced adenomas at a sensitivity of 100 ng Hb/mL. If we compare gFOBT and iFOBT by the OC method the sensor shows 3 times greater detection of advanced adenomas and carcinomas by the iFOBT method.

Source

 * with permission of the author taken from