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Hydrogen breath tests (HBT) are widely used for the diagnosis of small intestinal bacterial overgrowth (SIBO) in patients with irritable bowel syndrome (IBS). However, the conclusions drawn from these studies are highly controversial, and several discrepancies exist in the results. The aim of our study is to develop an alternative 13C-glucose breath test (13C-GBT) methodology by measuring high-precision 13CO2/13CO2 isotope ratios in exhaled breath to accurately diagnose SIBO using an optical cavity-enhanced CO2 isotope analyzer. In all, 118 diarrhea-predominant (IBS-D) patients diagnosed according to ROME III criteria underwent 13C-GBT and HBT following the ingestion of a test meal containing 50 mg 13C- enriched glucose with 50 g glucose. The excretion of CO2 enrichments and a cumulative percentage dose of 13C-recovered (c-PDR) significantly depleted (p < 0.001) for the positive SIBO patients (n = 25) compared to the patients with negative SIBO (n = 53) as diagnosed by HBT. A cut-o value of 5.47& at 45 min was indicative of a positive SIBO syndrome. Subsequently, a portion of IBS-D individuals (n=20) whose HBTs were negative but 13C-GBTs were positive, suggesting HBT often fails to diagnose SIBO when the patients may have "non-hydrogen-producing" bacteria. 13C-GBT also correctly diagnosed SIBO in patients (n = 20) within the "grey-zone" and during the preclinical phase of SIBO as opposed to HBT. The prevalence of SIBO with IBS-D in Indian population was estimated to be 45.7%. Our study demonstrates 13C-GBT, for the first time, as a clinically valid and sufficiently robust alternative diagnostic methodology for an accurate evaluation of SIBO in IBS-D patients and superior to HBT. [ABSTRACT FROM AUTHOR]