Patients present with chronic or recurrent abdominal pain or discomfort centred in the upper abdomen that is commonly related to eating. There may be a history of non-steroidal anti-inflammatory drug use and use of antacid medications to relieve the discomfort.
Peptic duodenitis is associated with acid injury and leads to a spectrum of histological mucosal changes that may be difficult to distinguish from that seen in coeliac disease.  For this reason, biopsies should be taken both in the duodenal bulb and in the second or third portion of the duodenum (relatively protected from peptic injury). Biopsies from the bulb and distal duodenum should be submitted to pathology in separate jars.
Crohn's disease can affect any part of the gastrointestinal tract, and symptoms may be extremely variable.
The classical findings on histological examination include granulomas, ulcerations, and acute and chronic inflammation often extending throughout all layers of bowel wall.
Tissue transglutaminase serology is usually negative and there should be no response to gluten withdrawal.
Giardiasis is a diarrhoeal illness caused by infection with a waterborne parasite, Giardia lamblia . A history of exposure to contaminated water may suggest the diagnosis. 
Multiple stool specimens usually reveal the parasite. Alternative methods for detection are antigen detection tests by enzyme immunoassays and detection of parasites by immunofluorescence. 
History may show conditions that alter intestinal anatomy, motility, and gastric acid secretion (such as use of proton pump inhibitors or anatomical disturbances in the bowel, including fistulae, diverticula, and blind loops created after surgery). 
The definitive investigation requires culture of jejunal fluid that grows in excess of 10⁵ bacteria/mL. Hydrogen breath testing may show malabsorption but is not very sensitive or specific for bacterial overgrowth. A trial of treatment with antibiotics for 1 week may give the diagnosis. 
In some children a clinical episode indistinguishable from acute gastroenteritis is followed by protracted diarrhoea. This may be related to prolonged rotavirus infection  or transient lactose intolerance.
Usually no investigations are required.
Eosinophilic enteritis may affect any part of the alimentary canal and can present with anaemia, diarrhoea, abdominal pain, and weight loss. Often no cause is identified, although nematode infections are often isolated. 
Diagnosis follows endoscopic or laparoscopic biopsy of the affected bowel with histology showing eosinophilic infiltrates. 
Tropical sprue is a disease that causes progressive villous atrophy in the small intestine that is similar to coeliac sprue. It is believed to be initiated or sustained by a still-undefined infection. The relapse rate is substantial in treated patients who remain in, or return to, endemic areas in the tropics. 
Antibiotic therapy with tetracyclines for 6 months normalises mucosal structure in the small intestine. 
CVID and related disorders have a history of recurrent infections.
Negative tissue transglutaminase serology and decreased immunoglobulin levels suggest immunodeficiency.
GVHD can occur with any organ transplantation but is most common after bone marrow transplantation. Patients have high-volume watery diarrhoea about 3 weeks after transplantation if GVHD is present. 
Endoscopic biopsy showing the presence of increased numbers of apoptotic epithelial cells in the intestinal crypts is diagnostic. 
May be clinically and pathologically indistinguishable from coeliac disease.
Olmesartan, an angiotensin-II receptor antagonist, has been associated with enteropathy. 
There have also been case reports with other angiotensin-II receptor antagonists and mycophenolate.  Use of non-steroidal anti-inflammatory drugs is also associated with lymphoplasmacytic infiltrate and partial villous atrophy.
Tissue transglutaminase serology is normal.
Symptoms remit once causative drug is stopped.
May share similar symptoms with coeliac disease, with improvement on a gluten-free diet. There should not be any villous atrophy.
Tissue transglutaminase serology remains normal. FBC and iron levels also remain normal, because the condition does not induce malabsorption. Small intestinal histology is normal. There is improvement of symptoms after 6 weeks (or less) on the gluten-free diet and recurrence with reintroduction of gluten.
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