Comparison of Minimum Inhibitory Concentration as Measured by Etests and Agar Dilution in Neisseria gonorrhoeae Isolates from North Carolina, 2018-2024.

BACKGROUND

In the United States, Neisseria gonorrhoeae is the second most common reportable sexually transmitted infection amassing 601,319 cases in 2023. Notably, N. gonorrhoeae has developed antimicrobial resistance, necessitating ongoing surveillance. In response to this threat, the U.S. Centers for Disease Control and Prevention has established antimicrobial resistance surveillance networks including Strengthening the US Response to Resistant Gonorrhea (SURRG) and the Antibiotic Resistance Lab Network (ARLN). The SURRG project performs testing using a gradient strip method, the Etest, whereas ARLN laboratories perform testing by agar dilution, for determining minimum inhibitory concentration (MIC) values for N. gonorrhoeae .

METHODS

We compared the concordance of MIC values obtained using the Etest gradient strip method at a SURRG site in North Carolina compared with ARLN using the agar dilution method for 3 antibiotics: azithromycin, cefixime, and ceftriaxone.The MIC values and the corresponding interpretations for each agent were analyzed according to the recommendations of the Clinical and Laboratory Standards Institute and the Centers for Disease Control and Prevention. The essential agreements were assessed.

RESULTS

Between January 2018 and December 2024, a total of 1951 N. gonorrhoeae had corresponding isolates from the ARLN laboratory available for comparison, of which 1892 had corresponding Etest and agar dilution results for all 3 antibiotics. We found high levels of MIC agreement between both testing methods for each antimicrobial agent tested (azithromycin 95%, cefixime 98%, ceftriaxone 95%) over 6 years including for strains isolated from distinct anatomical sampling sites.

CONCLUSIONS

The Etest method provides a robust alternative for accurately detecting antibiotic-resistant N. gonorrhoeae for public health surveillance, which may make them particularly useful in resource- or labor-limited settings.