Fecal occult blood test kits work via a colorimetric assay. ColoCARE and EZ Detect use the reagent tetramethylbenzidine which is present on the biodegradeable test pads of the kits.
Colorectal disorders such as cancer, cause blood to present on the outside of the stool. This blood disperses onto the surface of the toilet water. The heme portion of hemoglobin, present in blood, oxidizes the tetramethylbenzidine that is on the test strip and causes blue/green color change.
Hemoccult, another test kit, works by the same basic principle. Yet it works via the oxidation of guaiac by hydrogen peroxide. The heme portion of hemoglobin in blood catalyzes this reaction.