A pharmacometric grey zone reconciles high metronidazole resistance rates with bismuth quadruple therapy efficacy in Helicobacter pylori

Summary Background Metronidazole (MET) resistance in Helicobacter pylori (H. pylori) exceeds 50-60% globally, yet MET-containing bismuth quadruple therapy (BQT) achieves &gt90% eradication in MET-resistant infections. We hypothesise this discordance stems from a structural limitation of two-fold dilution: a pharmacometric grey zone between the 128 and 256 &microg/mL breakpoints where treatable isolates are systematically misclassified as high-level resistance. Methods In a real-world cohort of 4610 treatment-na&iumlve children (2019-2024), checkerboard assays determined the bismuth-MET synergy factor (SF). Population PK/PD modelling simulated gastric MET exposure (AUC<sub&gt0-24</sub&gt/MIC &ge 70) at 1 g, 1&middot5 g and 2 g daily doses, incorporated bismuth SF, and estimated MIC coverage against the European Committee on Antimicrobial Susceptibility Testing (EUCAST) population distribution. Findings were validated against published adult trials. Findings 56&middot4% (485/860) of isolates were MET-resistant (MIC &gt 8 &microg/mL). At [&le;] 1 g/day MET, BQT offered no advantage over triple therapy for resistant infections. At 1&middot5 g/day, the synergy threshold was surpassed, achieving &gt90% eradication in MET-resistant infections, with efficacy independent of baseline susceptibility status. PK/PD modelling showed 1&middot5 g/day MET plus bismuth (median SF = 8) extended coverage to ~180 &microg/mL. To align modelled coverage with the &gt90% eradication rate, the dominant subpopulation of nominally high-resistant isolates must have a geometric mean MIC well below 256 &microg/mL (centred near 150 &microg/mL), consistent with a mixture of two log-normal subpopulations and confirming systematic upward misclassification by two-fold dilution in the grey zone. Interpretation This grey zone is a structural blind spot of two-fold dilution, not random measurement error. The &plusmn1-dilution reproducibility limit renders the interval inherently unresolvable. We propose a three-tier strategy: omit routine MET susceptibility testing for treatment-na&iumlve patients on optimised BQT; reserve categorical testing for BQT failures; and explore genomic/metabolomic biomarkers beyond MIC paradigms in this enriched population. Funding None. Keywords: Helicobacter pylori; Metronidazole resistance; Bismuth quadruple therapy; Antimicrobial susceptibility testing; Pharmacokinetic/pharmacodynamic modelling; Pharmacometric grey zone