A simple method, easy to perform during an endoscopic procedure, fast and inexpensive, that allows detecting deficiencies in lactase, sucrase or maltase activities is presented. Briefly, method consists in placing a duodenal biopsy sample in an adequate vial containing lactose, sucrose or maltose solution during a few minutes, and then, adding a few drops of a glucose reactive from commercial origin. Presence of any enzymatic activity is demonstrated when released glucose from any of the disaccharides chosen reacts with the second reactive, turning solution to a red colour. Its utility is discussed and compared with other diagnostic methods.