What color does the guaiac reagent turn when oxidized in the presence of blood?

The guaiac reagent turns blue when oxidized in the presence of blood. This color change occurs due to the peroxidase-like activity of hemoglobin present in the blood, which catalyzes the oxidation of the guaiac resin. When blood is present, the reaction produces a blue compound, indicating a positive result for blood or hemoglobin.

The blue color development is significant in medical and forensic settings, particularly in fecal occult blood tests, where it is used to screen for hidden blood in stool samples as an indicator of gastrointestinal issues. It is essential for practitioners to recognize this color change, as it directly relates to the presence of blood, helping in diagnostic processes.