Background: It is helpful in clinical practice to predict the effects of eradication therapy on Helicobacter pylori. Aim: To develop a useful predictor of the response to metronidazole (MNZ)-containing second-line regimens by combining minimal inhibitory concentrations (MICs) of both amoxicillin (AMX) and MNZ, and the results of urea breath test (UBT) before the treatment. Methods: We enrolled 107 patients who showed eradication failure following first-line triple therapy with a proton pump inhibitor, AMX and clarithromycin. The eradication resistance index was defined as: [pre-treatment UBT result (‰)] × [AMX MIC (μg/ml)] × [MNZ MIC (μg/ml)]. Second-line eradication therapy with lansoprazole, AMX and MNZ was administered for 1 week. Eradication was confirmed by the UBT. Results: The eradication resistance index in subjects showing eradication failure and those showing successful eradication was 9.72 ± 6.63 and 1.25 ± 2.31, respectively (p < 0.001). When a cutoff value of 3 was used, the eradication resistance index predicted the response to therapy with a specificity of 93.8%, sensitivity of 81.8%, and accuracy of 92.5%. Conclusions: The eradication resistance index is a more useful predictor of response to MNZ-containing regimens for second-line treatment than only pretreatment UBT results or MNZ resistance.
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