Exploration of a Potential Amoxicillin MIC Threshold in Bismuth Quadruple Therapy for H. Pylori Infection in Southeastern China.

OBJECTIVE:The European Committee on Antimicrobial Susceptibility Testing (EUCAST) defines the amoxicillin resistance breakpoint at a minimum inhibitory concentration (MIC) of 0.125 μg/mL. This breakpoint is widely applied, and its clinical relevance has been validated in regions where amoxicillin-based triple therapy is the standard. However, in the context of China's predominant use of quadruple therapy, the applicability of this breakpoint remains unclear. This study aimed to collect Southeastern China resistance data, assess the impact of different amoxicillin MIC thresholds on the efficacy of amoxicillin-containing quadruple therapy, and to explore candidate amoxicillin MIC thresholds potentially associated with treatment outcomes. METHODS:Gastric mucosal specimens were obtained from patients undergoing gastroscopy between March 2024 and March 2025. A total of 4203 Helicobacter pylori isolates were successfully cultured and included for in vitro susceptibility testing. In vitro susceptibility testing of H. pylori isolates was performed using the agar dilution method, with MIC thresholds for amoxicillin set at 0.125 μg/mL, 0.25 μg/mL, 0.5 μg/mL, 1 μg/mL, and 2 μg/mL and other antibiotics (clarithromycin, metronidazole, levofloxacin) was performed on H. pylori isolates via agar dilution method. Patients treated with an amoxicillin-containing quadruple regimen (rabeprazole 10 mg twice daily + bismuth potassium citrate 220 mg twice daily + amoxicillin 1.0 g twice daily + furazolidone 100 mg twice daily or tetracycline 100 mg twice daily) were followed up. Eradication status was assessed by 13C-urea breath test 6-8 weeks post-treatment. Eradication rates were calculated using both intention-to-treat (ITT) and per-protocol (PP) analyses. χ2 test with Yates' continuity correlation and Fisher's exact tests were applied for group comparisons. Kappa statistics were used to evaluate the level of agreement between resistance classification under different MIC thresholds and clinical eradication outcomes. Prespecified stratified analyses were performed, and interaction terms were included in regression models to examine whether the association between amoxicillin MIC and eradication outcomes was consistent across treatment regimens and clinical subgroups. RESULTS:A total of 342 patients were included in the ITT analysis, and 330 patients were included in the PP analysis after excluding 14 patients due to adverse events or loss to follow-up. When the MIC breakpoint was set at 0.125 μg/mL or 0.25 μg/mL, no statistically significant difference was observed between resistant and susceptible groups in eradication rates (0.125 μg/mL: resistant ITT 88.57%, PP 90.29% vs. susceptible ITT 91.56%, PP 95.59%; 0.25 μg/mL: resistant ITT 86.89%, PP 88.33% vs. susceptible ITT 91.46%, PP 95.19%; all p > 0.05). At 0.5 μg/mL, eradication rates in resistant patients were significantly lower (ITT 77.42%, PP 80.00%) than those in susceptible patients (ITT 91.96%, PP 95.33%) (p < 0.05). At 1 μg/mL, eradication in resistant patients was 66.67% (ITT) and 72.73% (PP), significantly lower than susceptible patients (ITT 91.52%, PP 94.67%) (p < 0.05). The highest kappa coefficient (0.180) was observed at the 0.5 μg/mL threshold, indicating the strongest-albeit slight-agreement between resistance classification and treatment outcome. CONCLUSION:In patients receiving bismuth-containing quadruple therapy, an amoxicillin MIC threshold of 0.5 μg/mL appears to better reflect clinically relevant resistance than the EUCAST-defined breakpoint of 0.125 μg/mL. This threshold may serve as a more suitable candidate breakpoint for guiding empirical BQT in this region, although validation in larger, multicenter prospective studies is required.