Heme-negative stool is feces that tests negative for concentrations of heme, a component of the blood protein hemoglobin, according to MedBroadcast. Guaiac fecal occult blood tests use heme concentrations to determine the presence of occult blood in stool, which is an indicator of various gastrointestinal disorders.
Stool guaiac tests are the most common method of testing stools for heme concentrations, notes MedlinePlus. Heme-negative stools reduce the likelihood of disorders such as colon cancer, colon polyps, inflammation from GI tract infections, inflammatory bowel disease and various types of gastrointestinal tumors. However, certain substances, such as vitamin C, cause heme-negative readings even in instances where occult blood is present in the stool, according to the National Library of Medicine.