The pathology lab will do what is called a frozen section, where they freeze the biopsied tissue and take a very thin slice, examining it under a microscope. From that visual examination and their knowledge of what normal intestinal tissue should look like, they can detect such things as visual evidence of inflammation, granulomas (fairly unique to Crohns disease) and other abnormalities. In long standing CD they also look for cell dysplasia (changes in individual cell structure and appearance that could signal microscopic changes which could lead to cancer.) Basically the biopsy allows them to get down to cell level and see things that point to a more firm diagnosis. However, it is also important to keep in mind that the biopsies are only part of the diagnostic process -- the GI also considers clinical history (symptoms, etc,) and visual examination during the scope. The bottom line is that if you come to him with a history of cramping, diarrhea, maybe bleeding and/or weight loss . . . if he sees ulcerations or inflammation of strictures or other signs through the scope . . . and if the biopsies show inflammation and maybe granulomas, you can be pretty sure that you have Crohns. That said, if a biiopsy is taken from a non-inflamed skp area, or when you are not flaring, it may not show the same definitive signs. Like everything else ni medicine, it is a useful tool but not a sure thing.