Table 2.

Assessment of intestinal permeability [17,54].

Technique	General Principle	Test Site	Test Method	Limitations
Molecular Probes
Lactulose/mannitol	Oligosaccharides of different sizes	Small intestine	Urine	Time-consuming. Metabolised in the colon so limited application in assessing the large intestine (e.g., ulcerative colitis (UC)). Does not show permeation of bacterial components. Mannitol is contraindicated with blood transfusions.
Sucralose	Sucralose	Colon	Urine	Time-consuming. Does not show permeation of bacterial components.
Multi-sugar test	Sucrose, lactulose, sucralose, erythritol, rhamnase	Whole intestine	Urine	Time-consuming. Does not show permeation of bacterial components.
51Cr-EDTA	51Cr-EDTA crosses the intestinal barrier via the paracellular route and has similar physiological properties to oliogosaccharides.	Whole intestine	Urine	Invasive and complex detection method. Not readily available. Radioactivity. Impractical in clinical setting. Does not show permeation of bacterial components.
PEG4000/400	Polyethylene glycol, an inert molecule of different sizes.	Whole intestine	Urine	Time-consuming. The exact route of PEG is not well defined [70], thus implications in interpreting results. Does not show permeation of bacterial components.
Gadolinium-based MRI contrast agent [71]	Gadolinium (500–1000 Da)	Whole intestine	24-h urine collection	Lack of evidence in human studies. More expensive and may have higher toxicity than conventional sugars. Partial hepatobiliary elimination. Contraindicated in renal impairment.
Ussing chambers	Ion transport across the intestinal epithelium tissue sample is measured using a short circuit current.	Site-specific	Biopsy	Invasive and complex detection method. Ex-vivo. Lack of correlation between Ussing chamber and other permeability assays.
Imaging
Confocal laser endomicroscopy	Intravenously-administered fluorescent contrast is seen to leak through the small intestinal mucosa under real time endoscopy.	Terminal ileum, colon, duodenum	Endoscopy	Invasive. Time-consuming (average of 46.5 minutes [60]). Validated measurement scores include the Watson grade (semi-quantitative [60]) and confocal leak score (quantitative) [5]. Requires special training of the endoscopist. Does not show permeation of bacterial components.
Biomarkers of Intestinal Permeability
Claudin-3 [27]	Epithelial tight junction protein	NA	Urine	Limited data and lack of randomised trials.
Bacteria-Related Markers
Lipopolysaccharide (LPS) assay	Show endotoxemia from bacterial translocation due to barrier function failure.	Colon	Blood (portal venous)	Technical limitation in detecting low levels of LPS in the peripheral blood. Requires careful standardization of the measurement. Evidence of use in Inflammatory Bowel Disease (IBD).
Circulating endotoxin core antibodies	An indirect measure of translocation of bacterial products by quantifying immunoglobulins (IgG, IgM and IgA) against the inner core of endotoxin for acute phase of intestinal barrier damage and function [72].	Colon	Blood	Only study done on post-operative patients, not patients with chronic gastrointestinal disease. Evidence for use in IBD.
Plasma d-lactate	d-lactate is produced by the gut bacteria and translocated across the intestinal mucosa with barrier dysfunction.	Colon	Blood	False positive test with bacterial over growth. Limited use in critically ill patients (e.g., ischemic colonic injury, acute necrotizing pancreatitis).
Faecal butyrate concentrations	Butyrate is a barrier enhancing substance, modifying claudin-1 and -2 to preserve intestinal barrier function and preventing bacterial translocation.	Colon	Faeces	Poorly established. The test relies on the principle that butyrate as a single major component of the barrier function rather than a complex and interactive entity.
Bacteria-derived haemolysin	Toxin that impair the intestinal barrier.	Colon		Poorly established. Results are attributed to only haemolysin-producing bacteria.
Assessment of fatty liver disease	Inflammation and fatty liver disease result from translocation of bacteria and its products into the portal system.	Whole intestine	Imaging	Poor specificity.