In functional gastrointestinal disorders, it has become clear that duodenal mucosal permeability is related to the pathophysiology of the condition. Ussing chambersUssing Chamber have been conventionally used as a standard method for evaluating mucosal permeability. Other methods include the Lactulose/D-Mannitol TestLactulose/D-Mannitol Test, evaluation of mRNA or protein levels of intercellular junction molecules, and evaluation of intercellular spacesIntercellular space using an electron microscope or a high-magnification endoscope. It has also become possible to measure the impedanceImpedance and admittanceAdmittance values of the mucosa. Recently, instruments that evaluate admittance values in real time have also been developed. However, the relationship between duodenal mucosal permeabilityPermeability and symptom severity or therapeutic efficacy has not been sufficiently demonstrated. Various gastrointestinal mucosal permeability tests have been developed, and it is important to select the most appropriate testing method in full consideration of the situation (e.g., whether endoscopic examination is possible, whether tissue sampling can be performed, whether real-time evaluation is necessary).