Comparison of Disk Diffusion and Agar Dilution for Neisseria gonorrhoeae Susceptibility Testing and Update.

BACKGROUND

The aim of this study was to evaluate the validity of the disk diffusion for antimicrobial susceptibility testing of clinical Neisseria gonorrhoeae isolates, using the agar dilution as the reference standard. Four antimicrobial agents were assessed: penicillin, ciprofloxacin, ceftriaxone, and spectinomycin, with updated susceptibility data provided.

METHODS

A total of 185 Neisseria gonorrhoeae isolates, identified by 16S rRNA sequence analysis, were collected from clinical specimens obtained from patients in four general hospitals in Changsha, China, between 2014 and 2018. Antimicrobial susceptibility to the four agents was determined by both disk diffusion and agar dilution. Correlation and agreement between the two methods were analyzed by plotting inhibition zone diameters against their corresponding minimal inhibitory concentrations (MICs) and performing linear regression analysis.

RESULTS

By disk diffusion, the susceptibility rates were 1.1% (penicillin), 0.6% (ciprofloxacin), 98.4% (ceftriaxone), and 100% (spectinomycin), while by agar dilution the rates were 1.6% (penicillin), 0% (ciprofloxacin), 95.7% (ceftriaxone), and 100% (spectinomycin). The disk diffusion showed high categorical agreement with the agar dilution, with concordance rates of 98.9%, 99.4%, 96.2%, and 100% for penicillin, ciprofloxacin, ceftriaxone, and spectinomycin, respectively. Pearson correlation coefficients were -0.78, -0.65, -0.47, and -0.61 (all p < 0.001). Although discrepancies between the two methods were acceptable, further analysis is required to clarify discordant results observed for ceftriaxone in some isolates.

CONCLUSIONS

The disk diffusion remains a reliable qualitative approach for antimicrobial susceptibility testing of Neisseria gonorrhoeae, with results largely comparable to the agar dilution reference method.