Background: CLSI provides a guideline only for a agar dilution method of testing clarithromycin susceptibility for Helicobacter pylori. This study was to evaluate a disk diffusion method for clarithromycin and amoxicillin.

Methods: One hundred and forty clinical isolates of H. pylori isolated from May 2005 to May 2007 were tested by the CLSI agar dilution method and a disk diffusion method using 2μg (2CLR) and 15μg (15CLR) clarithromycin disks and 2μg (2AMX) and 10μg (10AMX) amoxicillin disks. The interpretation criteria used for the disk diffusion method were established by linear regression and error rate-bounded method for disk diffusion zone of inhibition (DDZ) compared to MIC.

Results: Resistance and intermediate rates to clarithromycin were 21.4% and 1.4%, respectively. A number of isolates with MIC 0.5, 1, and 2 (μg/mL) to amoxicillin were 7, 2, and 1, respectively. For 2CLR and 15CLR, the coefficients of determination (R²) between MIC and DDZ were 0.931 and 0.923 (P＜ 0.001), respectively, and the criteria for resistance/ susceptibility were 12/28 mm for 2CLR and 23/39 mm for 15CLR. For 2AMX and 10AMX, the R² between MIC and DDZ were 0.478 and 0.421 (P＜ 0.001), respectively, and the criteria for resistance with breakpoint of 2μg/mL were 21 mm for 2AMX and 32 mm for 10AMX. All isolates had DDZ＜60 mm with 2CLR and 2AMX, but 61.4% and 75.7% of the isolates had DDZ＜60 mm with 15CLR and 10AMX, respectively.

Conclusion: Excellent correlation and agreement between MIC and DDZ were found for clarithromycin and amoxicillin. With 2μg disks, the susceptibility breakpoints were 28 mm or less; thus, two disks could be tested in one plate. (Korean J Clin Microbiol 2009;12:30-36)