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 the Guaiacum tree[1]), 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 or, more frequently 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 the hydrogen peroxide 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.


  1. "Guidance for Industry and FDA Staff - Review Criteria for Assessment of Qualitative Fecal Occult Blood In Vitro Diagnostic Devices". United States Food and Drug Administration Office of In Vitro Diagnostic Device Evaluation and Safety. August 8, 2007.

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