Glucose breath test uncovers hydrogen- and methane-dominant subtypes of small bowel intestinal overgrowth in irritable bowel syndrome

<title>Abstract</title> <p> <bold>Background:</bold> Small intestinal bacterial overgrowth (SIBO) has been suggested in irritable bowel syndrome (IBS), but breath-test interpretation remains debated. We assessed hydrogen and methane responses during glucose breath testing (GBT) and whether an 80-minute window captured relevant pathological signals. <bold>Methods:</bold> We analysed 253 GBTs: 205 IBS patients and 48 healthy controls. IBS was classified as IBS-D, IBS-C, or IBS-M. Positivity was defined as hydrogen rise ≥12 ppm, methane rise ≥7 ppm, or both. <bold>Results:</bold> Pathological GBT was found in 5/48 controls (10%) and 63/205 IBS patients (31%). In IBS, 34/205 had H-SIBO, 15/205 M-SIBO, and 14/205 HM-SIBO. Hydrogen positivity was more common in IBS-D than non-IBS-D (28/87 vs 20/118; p = 0.013). Methane positivity was more common in IBS-C than non-IBS-C (16/40 vs 13/165; p &lt; 0.001). <bold>Conclusion:</bold> GBT identified hydrogen- and methane-associated IBS phenotypes within an 80-minute test window. </p>