Changes in the permeability of the small intestine can result in under-absorption of nutrients or the converse, over-absorption of the intestinal contents. Either condition can be described as malabsorption, although this term is more commonly used in the case of under-absorption of nutrients. A number of investigators have suggested that over-absorption syndromes (increased gut permeability) are significantly underdiagnosed. This may be important, for example, in cases of food intolerance, where derivatives of maldigested food may be absorbed through the gut wall and into the circulation, resulting in characteristic hypersensitivity symptoms.

The patient should fast for 3 hours before starting the test. Water intake during the first 2 hours of the 6 hour urine collection should be limited to 250 mL. Water consumption during the remainder of the test should be moderate.

The PEG test, which is a measure of mucosal permeability, should not be performed if the patient has gastroenteritis or is suffering from any other cause of intestinal hurry, as this will invalidate the urinary reference interval for the recovery of PEG.

The sample required for the gut permeability profile is a 6 hour urine collection after a 3 gram oral dose of PEG. A 20 mL aliquot of urine may be sent for analysis if the volume of the total collection can be accurately measured.