In disease states of the small intestine--for example,
gluten-sensitive enteropathy--there is an increased permeability to large molecules. This increased permeability extends to polar molecules of intermediate size such as
disaccharides, whereas small polar molecules are malabsorbed. A recently-developed oral test, based on the simultaneous administration of two test substances,
cellobiose (a
disaccharide) and
mannitol (a small polar molecule) has been used to investigate permeability in a variety of
gastrointestinal diseases, the result of the test being expressed as the ratio (
cellobiose/
mannitol) of the five hour urinary recoveries of the two probe molecules. Results for patients with
pancreatic insufficiency, intestinal bacterial overgrowth, primary
hypolactasia, ileocolic or colonic
Crohn's disease, and
ulcerative colitis were comparable with those in normal controls, whereas in 23 out of 24 untreated coeliacs, and five out of eight patients with
Crohn's disease involving the more proximal small bowel, the
cellobiose/
mannitol ratio was clearly abnormal. A study of its application as a screening procedure for coeliac disease showed that the test was both sensitive and accurate, with fewer false-positive and false-negative results than other recognised screening tests--namely, the
xylose test,
reticulin antibodies, and blood
folate estimations.