Fecal Occult Blood Test

Fecal occult blood test (FOBT) is designed to diagnose the occult bleeding into the gastrointestinal tract. Occult bleeding means only low blood losses when the blood in stool is not visible with the naked eye.


The basic principle of the test is detection of hemoglobin in a sample of faeces. Hemoglobin is a human protein, which is present in red blood cells. When the erythrocytes leak into the digestive tube, there is increased amount of hemoglobin in faeces.

The exact course of the examination depends of the used test kits. Ever test needs a small sample of stool, which is examined by some biochemical or immunochemical method that aims to detect hemoglobin. Older tests were only qualitative, i.e. they only told us that the result is positive or negative. Modern tests are quantitative, which means that they do not only evaluate the presence of hemoglobin, but also measure its amount. According to the value, it is possible to estimate, if the finding is significant and what could be its source.


There are more types of the FOBT tests. The older and less reliable needed special dietary restrictions few days before the test when the patient should avoid foods such as salami, sausages, raw meat, spinach, turnips and others as they may cause a false positivity of the test. Modern FOBT have no dietary restriction. The FOBT should not be performed in people with acute diarrhea and in women during menstruation.


FOBT is important in diagnosis of intestinal polyps and early colorectal cancer. The polyps are usually benign tumors, but when left untreated, they may slowly progress into a malignant disease. The thing is that all these lesions are asymptomatic, but they may cause very mild bleeding into the stool invisible to human eye – the aforementioned occult bleeding. By detection of the blood, we may suspect the presence of a larger polyp or even the colorectal cancer.


The examination is only indirect. When it is positive, it should be followed by further examination of the colon, usually by colonoscopy.


The FOBT is quick, cheap, totally non-invasive and its modern variants are pretty reliable. These advantages make FOBT an ideal examination method for population screening of colorectal cancer, which is performed in many developed countries.


Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Contact: jiri.stefanek@seznam.cz
 Sources: basic text sources