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Introduction Intestinal methanogen overgrowth (IMO) is an increasingly recognised condition caused by methane-producing archaea in the small intestine. To detect methanogenesis, combined hydrogen-methane breath testing (HMBT) is required. Many testing centres however still use hydrogen-only testing. IMO treatment regimens differ from those for SIBO and usually incorporate the aminoglycoside neomycin.Methods 292 consecutive patients referred to at Royal Berkshire Foundation Trust (RBFT) from May 2024 to December 2025 were included in this study. Patient characteristics, referral symptoms and test results were analysed. All breath tests were performed on a Laborie Gastrogenius HMBT portable desktop monitor.Results 67 patients (23.0%) were methane-positive on HMBT, suggesting a diagnosis of IMO. 54 patients (18.5% of the total cohort) of these were methane positive only, and so would have had missed diagnoses of IMO if tested on a hydrogen-only machine. 99 patients (33.9%) were hydrogen-positive, suggesting small intestinal bacterial overgrowth (SIBO), including 13 patients who were dual hydrogen-methane positive.Patients with methane-positive tests were significantly older than those with hydrogen-positive tests (mean difference 7.7 years, p=0.0056). Symptom profiles did not significantly differ between IMO and SIBO groups.Conclusions Our study demonstrates a substantial proportion of patients with abnormal breath tests who would not have been identified using hydrogen-only breath testing. Patients with IMO were older than those with SIBO, while symptoms overlapped significantly. Given the therapeutic implications of methane detection, centres currently using hydrogen-only breath testing should consider adoption of combined HMBT.Abstract P419 Figure 1Stacked bar chart showing hydrogen-positive, methane-positive-only and dual-positive breath tests as a proportion of all abnormal results