Assessment of metronidazole susceptibility in Helicobacter pylori: Statistical validation and error rate analysis of breakpoints determined by the disk diffusion test

Metronidazole susceptibility of 100 Helicobacter pylori strains Has assesse d by determining the inhibition zone diameters by disk diffusion test and t he MICs by agar dilution and PDM Epsilometer test (E test). Linear regressi on analysis was performed, allowing the definition of significant linear re lations, and revealed correlations of disk diffusion results with both E-te st and agar dilution results (r(2) = 0.88 and 0.81, respectively ). No sign ificant differences (P = 0.84) were found between MICs defined by E test an d those defined by agar dilution, taken as a standard. Reproducibility. com parison between E-test and disk diffusion tests showed that they are equiva lent and with good precision. Two interpretative susceptibility schemes (wi th or without an intermediate class) were compared by an interpretative err or rate analysis method. The susceptibility classification scheme that incl uded the intermediate category was retained, and breakpoints were; assessed for diffusion assay with 5-mu g metronidazole disks. Strains with inhibiti on zone diameters less than 16 mm were defined as resistant (MIC > 8 mu g/m l), those with zone diameters equal to or greater than 16 mm but less than 21 mm were considered intermediate ( 4 mu g/ml < MIC less than or equal to 8 mu g/ml), and those with zone diameters of 21 mm or greater were regarded as susceptible (MIC 4 mu g/ml). Error rate analysis applied to this classi fication scheme showed occurrence frequencies of 1% for major errors and 7% For minor errors, when the results were compared to those obtained by agar dilution. No very major errors were detected, suggesting that disk diffusi on might be a good alternative for determining the metronidazole sensitivit y of H, pylori strains.