LAUSR

OBJECTIVES Antimicrobial resistance to clarithromycin and metronidazole significantly affects the cure rate of standard therapies for Helicobacter pylori infection. We tested whether different MIC levels of resistance to these antibiotics play a role in therapeutic efficacy. METHODS This was a post hoc analysis of data from a therapeutic trial in which patients with antibiotic susceptibility testing (Etest) received first-line sequential therapy. The level of antibiotic resistance was classified according to MIC values into low (MIC from >0.5 to ≤8 for clarithromycin, and from >8 to ≤32 for metronidazole) and high (MIC from >8 to 256 mg/L for clarithromycin, and from >32 to 256 mg/L for metronidazole). RESULTS Data from 1006 patients were included. There were 520 (51.7%) patients with susceptible strains, 136 (13.5%) with clarithromycin-resistant strains, 144 (14.3%) with metronidazole-resistant strains and 206 (20.5%) with clarithromycin-resistant and metronidazole-resistant strains. In the presence of double resistance, the cure rate was still high (38/41, 92.7%) when MIC levels were low and it was reduced (94/112, 83.9%) only when MIC levels of both antibiotics were high. The cure rates did not significantly differ between patients with single antibiotic-resistant strains, irrespective of MIC values, and those with susceptible strains. CONCLUSIONS We found that MIC levels of resistance to either clarithromycin or metronidazole play a role in H. pylori therapy outcome and that bacterial resistance becomes relevant in vivo when clarithromycin-resistant and metronidazole-resistant strains have high MIC values for at least one of these antibiotics.