Stool reducing substances test is used to diagnose lactose intolerance and some rare metabolic abnormalities .Frank blood can easily be seen even by your own self in the stool but sometimes blood is present in the stools but cannot be seen with the naked eye, called occult blood. This can be due to several conditions in your digestive tract such as polyps, cancer, ulcers, fissures, diverticular disease etc. Certain foods have to be avoided 3 days prior to the test like meat, raw fruits & vegetables as they can affect the test result.