Performance characteristics of newer MIC gradient strip tests compared with the Etest for antimicrobial susceptibility testing of Neisseria gonorrhoeae

Abstract

For Neisseria gonorrhoeae susceptibility testing, Etest, comparable to agar dilution, is frequently used. In recent years, newer MIC gradient strip tests have been commercialized. However, these tests have not been appropriately evaluated for gonococci. We evaluated the sensitivity, specificity, accuracy, quality, availability of antimicrobials and cost of the MIC Test Strip (Liofilchem), M.I.C.Evaluator (Oxoid) and Ezy MIC Strip (HiMedia), compared to the reference Etest (bioMérieux), for gonococcal susceptibility testing. The MICs of eight antimicrobials in 103 gonococcal international reference strains (n = 29) and clinical isolates (n = 74) were examined. Coefficient of determination (R ), complete agreement, essential agreement, SIR categorical agreement, sensitivity, specificity and accuracy were calculated. R of the MICs for the antimicrobials ranged between 0.674-0.996, 0.617-0.993, and 0.643-0.994 for the MIC Test Strip, M.I.C.Evaluator strips and Ezy MIC Strips respectively. The essential agreement (SIR categorical agreement) was 99.6% (88.6%), 100% (87.1%) and 93.0% (83.1%) respectively. M.I.C.Evaluator strips for gonococcal key antimicrobials were lacking and the Ezy MIC Strips showed an inconsistent accuracy, quality and some strips were contaminated. The Liofilchem MIC Test Strips had limitations, but might be relatively accurate alternatives to Etest for gonococci. Strict quality assurance (at manufacturing and testing laboratory), including quality controls, are required.

Variability in Azithromycin Susceptibility Results for Neisseria gonorrhoeae Obtained Using Gradient MIC Strip and Agar Dilution Techniques

“…However, the mean and modal MICs obtained by both gradient strip methods were higher than those determined by the agar dilution MIC method. In consequence, agreement between the two gradient strip methods was higher (88%) than that obtained for each method compared with the agar dilution MIC method, consistent with a previous study which found good agreement when comparing the Etest and the MIC test strip (7). This finding is notable as it suggests that future comparison studies of gradient MIC strips should continue to include the agar dilution MIC method as the reference method.…”

“…gonorrhoeae, but it is labor intensive, is time consuming, and does not present results in a clinically relevant time frame (5). There are now several azithromycin gradient MIC strips available, and the limited comparison studies available demonstrate promising equivalence of these methods (5)(6)(7).…”

Evaluation of the accuracy of molecular assays targeting the mutation A2059G for detecting high-level azithromycin resistance in <em>Neisseria gonorrhoeae</em>: a systematic review and meta-analysis

“…Therefore, though the MICs estimated by different laboratories are comparable, their values may vary by one or more twofold dilution due to slight technical differences, hence affecting the clinical interpretation 37. In terms of the E-test method, it is costly, requiring the use of experimental materials that are very expensive because of manufacturer’s patent protection, and these materials were not available in some areas 38. On the other hand, the molecular assays (PCR and WGS) detecting the mutation A2059G could be an alternative method for identifying isolates with high-level azithromycin resistance; however, they have not been developed into a commercial diagnostic kit for clinical use nowadays.…”

External quality assessment (EQA) of Neisseria gonorrhoeae antimicrobial susceptibility testing in primary laboratories in Germany

“…Agar dilution for quantitative determination has been replaced by the minimal inhibitory concentration (MIC) gradient strip test methodology in the last years. This is an easier and less time consuming method providing precise and reliable results [13]. However, MIC gradient strip tests are more costly than classical, less accurate test methods such as disc diffusion techniques.…”

“…For a few years now, different companies have been offering MIC strip tests for antibiotic susceptibility testing. However, it has been demonstrated that the choice of gradient strip test manufacturers can influence testing outcomes [13], as do bacterial growth conditions, i.e. the choice of media or pH [9].…”