The most appropriate diagnostic test to perform next is tissue transglutaminase serology. This patient fulfills the criteria for irritable bowel syndrome with diarrhea (IBS-D) and has no accompanying alarm features. Therefore, the diagnosis of IBS-D can be confidently made with minimal testing. Such testing should be limited to studies assessing for organic diseases that are more prevalent in patients with IBS-D than in the general population. A meta-analysis and systematic review of 14 studies including 4204 patients (54% meeting criteria for IBS) found that patients with IBS are three to four times more likely to have positive celiac antibodies and are more than four times more likely to have biopsy-proven celiac disease than controls. Therefore, screening for celiac disease with tissue transglutaminase testing is warranted.

This patient has no alarm features to warrant a colonoscopy or sigmoidoscopy. Furthermore, a study of more than 900 adults (466 with IBS and 451 controls undergoing screening colonoscopy) found the prevalence of structural abnormalities on colonoscopy, including polyps, colorectal cancer, and inflammatory bowel disease, to be no greater in patients with IBS than in controls.

Lactose hydrogen breath testing would be a consideration only if there was a reported connection between the ingestion of milk and bowel symptoms. This patient reported no change when eliminating milk from her diet.

The prevalence of abnormal thyroid-stimulating hormone (TSH) findings is no greater in patients with IBS than in the general population. Furthermore, there is no established causality between abnormal TSH results and the symptoms reported in IBS.