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Background: Neisseria gonorrhoeae is a sexually transmitted bacterial pathogen that continues to evolve to become resistant to known antibiotics. Current treatment recommendations by the Centers for Disease Control and Prevention (CDC) are limited to either ceftriaxone or cefixime in combination with azithromycin. In preparing for potential emergence or importation of such resistant isolates in the US, the CDC recommends that clinical laboratories maintain or develop protocols to assess antibiotic susceptibly for N. gonorrhoeae. Methods that determine minimum inhibitory concentrations (MICs) would be best since interpretative breakpoints have yet to be established when testing N. gonorrhoeae against these antibiotics. This study examines the intra- laboratory variability of using the Etest method to provide consistent MIC values for gonorrhea.
Methods: A total of 200 clinical N. gonorrhoeae isolates, 100 paired duplicates, were tested by 8 public health laboratories for antibiotic susceptibility to ceftriaxone, cefixime and azithromycin using Etest strips. The laboratories were not aware that the isolates were paired duplicates. The MIC values were transformed to a logarithmic scale to assess correlation between the paired isolates. A correlation coefficient was calculated to assess intra-laboratory reproducibility.
Results: Laboratories inoculated GC agar base medium with approximately 108 CFU/mL and placed Etest strips onto the medium. The correlation coefficient for intra-laboratory reproducibility ranged from 0.39 to 0.93 for ceftriaxone, from 0.08 to 0.82 for cefixime, and from 0.40 to 0.91 for azithromycin.
Conclusions: There was considerable variation in laboratory reproducibility of Etest MIC results for antibiotics relevant for the treatment of N. gonorrhoeae. The development of standard laboratory protocols and training may help to improve the intra-laboratory performance of the Etest for N. gonorrhoeae antibiotic susceptibility testing. To improve standardization in testing, the CDC is working with the Association of Public Health Laboratories to develop an Etest training module for N. gonorrhoeae.