The most appropriate diagnostic test to perform next is testing for serum tissue transglutaminase antibodies. This patient has chronic diarrhea with clinical evidence of steatorrhea, the differential diagnosis of which includes small-bowel mucosal diseases, pancreatic insufficiency, small intestinal bacterial overgrowth, and lymphatic obstruction. Celiac disease is a chronic inflammatory enteropathy caused by an immune-mediated reaction to gluten and gliadins and is present in 0.5% to 1% of the U.S. population. The chronic diarrhea, abdominal pain, and malabsorption present in this patient suggest this diagnosis, and the initial step is testing for serologic markers for the disease. IgA tissue transglutaminase antibody is the screening test of choice for celiac disease and is both sensitive and specific; initial testing for celiac disease should be done while the patient is consuming gluten.
While anti–Saccharomyces cerevisiae antibodies (ASCA) have been proposed as a serologic method for differentiating Crohn disease from ulcerative colitis, they are neither adequately sensitive nor specific and can lead to false-positive results if used as a screening test for gastrointestinal symptoms. There is no role for ASCA in the evaluation of a patient with suspected celiac disease.
Endoscopy (flexible sigmoidoscopy or colonoscopy) can be an important test in evaluating selected patients with chronic diarrhea, but it would not be the most appropriate next study in this patient with diarrhea with a high pretest probability of celiac disease given his history of type 1 diabetes mellitus with clear evidence of malabsorption. Flexible sigmoidoscopy in particular would only visualize the distal aspects of the colon, and because colonic disorders do not cause malabsorption, this test would not be diagnostic in this patient. A positive tissue transglutaminase test and confirmation of celiac disease as the cause of diarrhea in this patient would make colonoscopy unnecessary and avoid the potential risks of the procedure.
Upper endoscopy with small-bowel biopsies is performed in patients with a consistent clinical picture and positive serologic studies to confirm the diagnosis of celiac disease. Characteristic biopsy findings include intraepithelial lymphocytosis, crypt hyperplasia, and villous blunting. However, upper endoscopy with biopsies would not be indicated as an initial study in patients suspected of having celiac disease.
Video capsule endoscopy can visualize the small-bowel mucosa and might show nonspecific changes in a patient with suspected celiac disease; however, it is not indicated as an initial diagnostic study before serologic testing or for confirmation if the patient is a candidate for routine upper endoscopy with biopsy.