Guaiac test is used for

• Based on the property of an organic compound, guaiac, to turn blue after oxidation by oxidants, peroxidases, or the pseudoperoxidase of hemoglobin in the presence of an oxygen donor such as hydrogen peroxide. • Guaiac tests are more sensitive for detecting bleeding from the lower than from the upper GI tract because hemoglobin (and its pseudoperoxidase activity) is degraded as it moves down the GI tract. • Fecal hemoglobin levels must exceed 10 mg/g (blood loss of 10 ml daily) for 50% of Hemoccult II tests to be positive.