The most appropriate management is a careful dietary review. Celiac disease is an immunologic response to dietary gliadins in patients who are genetically at risk as deemed by the presence of HLA-DQ2 or HLA-DQ8. All patients with celiac disease should adhere to a gluten-free diet by avoiding wheat, barley, and rye. Because of cross-contamination with other cereal grains, oats should be avoided for the first year and should only be introduced if the patient is doing well clinically. In patients whose symptoms are recurrent or do not respond to a gluten-free diet, gluten ingestion (either surreptitious or inadvertent) is the most likely explanation. In many cases, patients believe they are being compliant with a strict gluten-free diet, but careful review by an experienced dietitian reveals inadvertent gluten ingestion. Associated conditions that may account for recurrent diarrhea are microscopic colitis (70-fold increased risk), lactose malabsorption, small intestinal bacterial overgrowth, pancreatic insufficiency, inflammatory bowel disease, refractory celiac disease, or enteropathy-associated T-cell lymphoma. A careful review of diet and potential nondietary sources of gluten exposure (such as medications, lipstick, and toothpaste) should be explored before performing additional testing.

A CT scan would be helpful to exclude enteropathy-associated lymphoma or adenocarcinoma; however, these diagnoses are extremely uncommon, even in patients with celiac disease. Dietary indiscretion is a much more likely cause.

Colonoscopy with biopsies would be useful to exclude microscopic colitis, which is associated with celiac disease, but a second diagnosis should be pursued only after ruling out gluten ingestion.

Upper endoscopy with small-bowel biopsies would be helpful to evaluate for active celiac disease, but it would not determine whether gluten ingestion is the cause of this patient's symptoms.