Recently, I attended the virtual, global ECCMID 2021 conference and was struck with the quality and sheer volume of information presented. After attending a jam-packed workshop entitled ‘AST Testing with EUCAST Methods and Breakpoints’ many of the speakers emphasized that doing AST today takes more skills than ever before, and after this session I can see why. Managing the testing, generating quality data, and interpreting the results is a lot to handle.
As the speakers demonstrated, the minimum inhibitory concentration (MIC) can vary depending on the solvent used to prepare the antimicrobial agent, and the potency, solubility, and stability of the agent. For mycobacteria AST, no standardized MIC methods are available. There are also challenges with the new antituberculous agents due to the presence of the solvent they are dissolved in (DMSO). For dermatophytes, the antifungals Nidulafungin and Micafungin are highly hydrophobic, and bind to plasticware, and the MIC actually increases with the number of pipette tip changes when serial dilution is performed. Optimizing AST for the polymyxins like Colistin is also challenging due to its poor agar diffusion in agar, and its binding to plasticware. There could also be species-specific variation, since the EUCAST strain collection is more varied compared to that of CLSI.
The ‘ATU’ (area of technical uncertainty) is a term coined by EUCAST to warn labs of uncertain interpretation of AST results. The ATU is defined by one or more MIC-value (in the case of MIC determination) or by one or more inhibition zone diameter (in the case of disk diffusion). The way the warning is handled depends on the situation and can be influenced by sample type, antimicrobial agent, and infecting organism. For more about ATU, review the guidance here.
If uncertainty is creeping into your workflows, we are here to help. We can optimize the antimicrobials used in susceptibility testing to help with some of the issues that can affect performance of your AST systems. Visit our AST Services page to learn more.
References
Soares A et al (2020) Area of technical uncertainty for susceptibility testing of amoxicillin/clavulanate against Escherichia coli: analysis of automated system, Etest and disk diffusion methods compared to the broth microdilution reference. Clin. Microbiol. Infec. 26 (12):1685.e1-1685.e6
Vasso S (2017) Susceptibility testing for the Polymyxins: Two steps back, three steps forward? J. Clin. Microbiol. 55(9):2573-2582