Diagnostic Evaluation of Small Intestinal Microbial Overgrowth: A Cross-Sectional Comparison of Glucose and Lactulose Breath Tests

<b>Background/Objectives</b>: Small intestinal microbial overgrowth (SIMO), including both small intestinal bacterial overgrowth (SIBO) and intestinal methanogen overgrowth (IMO), is commonly diagnosed using non-invasive breath tests, whose diagnostic performance and criteria remain inconsistent. This study aimed to assess SIMO prevalence using lactulose (LBT) and glucose breath tests (GBT), compare their diagnostic yields for SIBO and IMO, analyze associated gas profiles, clinical features, risk factors, and evaluate the diagnostic accuracy of a simplified fasting methane criterion for IMO. <b>Methods</b>: Cross-sectional study conducted on 564 outpatients (75.7% female) with suspected SIMO. Patients underwent LBT (n = 275), GBT (n = 289), or both (n = 47). <b>Results</b>: SIMO was diagnosed in 26.8% of patients. LBT identified significantly more SIMO than GBT (37.5% vs. 16.6%, <i>p</i> < 0.01), particularly for SIBO (24.4% vs. 4.8%, <i>p</i> < 0.01), while IMO detection was comparable (9.8% vs. 10.7%). Mixed overgrowth (dual SIBO/IMO positivity) showed a borderline trend favoring LBT. Methane peaks occurred significantly earlier than hydrogen in both BTs. Clinical symptoms did not significantly differ between SIMO subtypes or between test-positive and test-negative groups. The simplified fasting methane criterion showed limited diagnostic accuracy for IMO making it inadequate as a standalone diagnostic tool, requiring further validation before clinical implementation. <b>Conclusions</b>: GBT is the more reliable test for SIMO diagnosis due to LBT's lower specificity. Clinical symptoms alone were not predictive of SIMO subtypes, while the different gas profile suggests a distinct spatial distribution of microbial populations with a higher proximal concentration of methanogenic Archaea.