Lactulose Hydrogen–Methane Breath Testing for Clinical Characterization of Small Intestinal Bacterial Overgrowth in Adults with Functional Bloating

Functional bloating is a common disorder of gut–brain interaction with heterogeneous symptom patterns and uncertain underlying mechanisms. Small intestinal bacterial overgrowth and intestinal methanogen overgrowth have both been implicated in gas-related symptoms; however, the clinical utility of combined lactulose hydrogen–methane breath testing for phenotypic characterization in patients with functional bloating remains incompletely defined. This cross-sectional observational study evaluated whether lactulose hydrogen–methane breath testing could distinguish clinically relevant subgroups among symptomatic patients with functional bloating. Ninety adults with functional bloating underwent standardized lactulose hydrogen–methane breath testing with serial sampling over 120 min. Demographic characteristics, symptom duration, bloating severity, abdominal pain, bowel habit patterns, and breath gas profiles were recorded. Breath test results were categorized as hydrogen-positive, methane-positive, mixed-positive, or negative according to predefined interpretation criteria. Breath testing was positive in 42 of 90 patients, including 24 hydrogen-positive, 12 methane-positive, and 6 mixed-positive cases. Patients with positive breath tests exhibited greater bloating severity than those with negative breath tests. Methane-positive and mixed-positive groups demonstrated a higher prevalence of constipation-related features, whereas the hydrogen-positive group showed greater overlap with mixed bowel habits. These findings demonstrate that combined lactulose hydrogen–methane breath testing can identify clinically meaningful gas-production phenotypes in adults with functional bloating and may support symptom-based clinical stratification.