Comparison of gentamicin MICs by agar dilution and Etest for clinical isolates of Neisseria gonorrhoeae

BACKGROUND: In South Africa, Neisseria gonorrhoeae (NG) is the predominant cause of male urethritis syndrome (MUS). The national MUS treatment guidelines recommend gentamicin as salvage therapy for ceftriaxone treatment failures. We ascertained and compared gentamicin MICs obtained by agar dilution and Etest for clinical isolates of NG. METHODS: Gentamicin MICs for NG culture isolates obtained from 272 MUS cases in 2017 were determined using agar dilution, as per CLSI agar dilution methods, and Etest® (bioMérieux, Marcy-l'Étoile, France). Previously published interpretive criteria were used: MIC ≤4 mg/L, susceptible (S); MIC 8-16 mg/L, intermediately resistant (IR); and MIC ≥32 mg/L, resistant (R). WHO 2008 NG reference strains were used as comparison standards. RESULTS: Gentamicin agar dilution versus Etest MIC results (mg/L) were as follows: MIC50 = 16 versus 4; MIC90 = 16 versus 8; minimum MIC = 4 versus 1; and maximum MIC = 32 versus 16. Interpretive categories for agar dilution versus Etest were as follows: S, 4.4% versus 86.8%; IR, 86.0% versus 13.4%; and R, 9.6% versus 0%. The gentamicin MIC50 by agar dilution was significantly higher than by Etest (sign test P value <0.001); overall MIC agreement was 7.4% [kappa statistic (κ) = -0.014 (95% CI -0.039 to 0.010)]. Correlation with expected MICs for WHO reference strains was consistently better with Etest than with agar dilution. CONCLUSIONS: There was a significant discordance between NG gentamicin MICs by agar dilution versus Etest. NG gentamicin AST methodology must be standardized and interpretive criteria established to optimize the monitoring of susceptibility trends.