Table 1.

Summary table of the different methods to assess intestinal permeability.

Method	Principle/description	Application	Advantages/disadvantages
•Ussing chambers	- Measures the epithelial permeability to ions by assessing TEER - Measures the epithelial permeability to molecules of different molecular sizes to assess the paracellular and transcellular pathways	Used to assess different aspects of the epithelial integrity ex vivo in endoscopic biopsies or resection specimens of different regions of the GI tract	A: gold standard technique for assessing epithelial integrity (intestinal permeability) D: trained researchers and investment in the equipment. D: time and labor-intensive and are not available in most labs
•Differential urinary sugar excretion (most commonly lactulose-mannitol but can include other sugars like sucrose, sucralose, rhamnose)	- Measures the paracellular permeability of the epithelium to different sugars after drinking a sugar cocktail. Sucrose discriminates the paracellular permeability of the gastroduodenal region, lactulose/mannitol the small intestine permeability, and sucralose the colonic/whole GI tract permeability	Used to assess the permeability of the GI tract by measuring in urine the concentrations of the different sugars administered at different fractions of time	A: low cost of the test, a large number of subjects can be included D: determinations of sugar concentrations in urine require investment in a HPLC or LC-MS equipment and trained researchers. D: only measures the paracellular permeability D: time and labor-intensive and not readily available in most labs
•Confocal laser endomicroscopy	- Measures the leakage of fluorescein after intravenous administration visualized using a confocal probe equipped with a 488 nm laser	Used to assess three parameters: the enhancement of the gaps between epithelial cells, leakage of fluorescein into the lumen, and cell shedding	A: relatively easy to perform D: trained researchers and investment in the equipment D: assesses the permeability from the basolateral compartment to the lumen
•Mucosal impedance testing	- Measures mucosal impedance (the equivalent of the resistance for an alternating current) for several seconds through a probe inserted in the biopsy channel of a standard endoscope	Used and validated to assess epithelial integrity in the esophagus. of GERD patients, patients with eosinophilic esophagitis, and other esophageal disorders	A: can be performed during a routine endoscopy D: short assessment of the epithelial integrity (max. 90 s) D: values are the reflection of the epithelial permeability to ions D: validated in the esophagus but not in other parts of the GI tract
•Serum biomarkers: LPS; LBP, sCD14, I-FABP, zonulin	- Determination of serum/plasma concentrations using ELISA	Used to determine blood protein concentrations indicative of bacterial translocation, epithelial damage…	A: reasonable cost and relatively easy to perform in large amounts of samples D: some commercially available ELISAs do not detect the target protein or detect related molecules D: validation of most of these biomarkers against standard permeability measurements and proper clinical validation studies are lacking

GI, gastrointestinal; A, advantages; D, disadvantages; GERD, gastro-esophageal reflux disease; HPLC, high performance liquid chromatography; LC-MS, liquid chromatography–mass spectrometry; LPS, lipopolysaccharide; LBP, LPS binding protein; I-FABP, intestinal fatty-acid binding protein; sCD14, soluble CD14; ELISA, enzyme-linked immunosorbent assay.