Stool guaiac test

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Guaiac cards and bottle of developer.

A stool guaiac test detects the presence of fecal occult blood.

It involves feces, a thick piece of paper attached to a thin film coated with guaiac (which comes from a plant), and hydrogen peroxide. Either the patient or medical professional smears a small fecal sample onto the film, drips one or two drops of peroxide on the film, and observes for a color change.

The fecal sample can be obtained either by wiping soiled toilet tissue on the film. In medical settings, it's obtained either with a gloved finger (with a digital rectal exam) or with a wooden stick dipped into stool. Only a small sample for smearing is necessary; a mound of stool would actually impede an accurate test.

When peroxide (a liquid) is dripped onto the guaiac, it oxidizes the guaiac causing a color change. This oxidation occurs very slowly. Heme, a component of hemoglobin found in blood, catalyzes this reaction, giving a result in about 2 seconds. Therefore, a positive test result is one where there is a quick color change of the film.