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Fecal occult blood test
Make it clearer
The fecal occult blood test looks for blood in stool when there is so little of it that it cannot be seen with the naked eye: that is why it is called occult. Its main use is colorectal cancer screening, meaning looking for cancer in people who do not yet have symptoms.
The most widely used version does not look for whole blood but for heme, the iron-carrying part of hemoglobin. Heme has pseudoperoxidase activity, a fancy way of saying that it behaves like an enzyme that splits hydrogen peroxide and releases oxygen from it. That freshly released oxygen oxidizes a chromogen, a substance that changes color when it is oxidized. The test does not see the blood: the heme produces a color that can be seen.
In the guaiac test, guaiac being a natural resin, the oxygen oxidizes the α-guaiaconic acid the resin contains. That reaction yields a quinone, a chemical structure that rearranges internally until it becomes a blue dye.
In brief
The fecal occult blood test detects microscopic blood in stool and is used mainly to screen for colorectal cancer.
The predominant method targets heme: its pseudoperoxidase activity releases nascent oxygen from hydrogen peroxide, and that oxygen oxidizes a chromogen.
The test does not see the blood: the heme produces a color that can be seen.
With guaiac, the released oxygen oxidizes α-guaiaconic acid, and the reaction forms a quinone that rearranges into a blue dye.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingFecal Occult Blood Test, StatPearlsPublished
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