Multilaboratory Testing of Two-Antifungal Drug Combinations Against Candida species and Aspergillus fumigates: Utility of 100% Inhibition as an End-point for Synergistic Measurement

P. Ren 1, M. Luo2, S. Lin2, M. Ghannoum3, N. Isham3, D. Diekema4, M. Pfaller4, S. Messer4, S. Lockhart5, N. Iqbal5, V. Chaturvedi1; 1Wadsworth Ctr., Albany, NY, 2Bureau of Environ. & Occup. Epidemiol., NYSDOH, Albany, NY, 3Case Western Reserve Univ. Hosp., Cleveland, OH, 4Univ. of Iowa Coll. of Med., Iowa City, IA, 5CDC, Atlanta, GA


In this study, four laboratories performed antifungal combination testing using checkerboard microbroth dilution method for two Candida species and two Aspergillus fumigatus isolates, each with multi-drug resistance features. C. krusei ATCC 6258 was used as QC strain. 96-well custom-made plates containing checkerboard pairwise combinations of 8 antifungal drugs amphotericin B (AMB), anidulafungin (AND), caspofungin (CSP), micafungin (MCF), fluconazole (FLC), itraconazole (ITC), posaconazole (PSC), and voriconazole (VRC) were used as testing device. The experiment was repeated once by all participating laboratories. MIC at both 50% and 100% inhibition levels were recorded visually. Summation fractional inhibitory concentration indices (FICI) were calculated and interpreted into synergy (FICI ≤ 0.5), indifference (FICI > 0.5 and ≤ 4.0), and antagonism (FICI > 4.0). Majority FICI values indicated indifferent drug interactions. No consistent antagonistic drug interaction was reported for any fungal isolate against any drug combination. Synergy was found for CSP-PSC against C. albicans 20464.007, AMB-PSC against C. glabrata M1409, PSC-VRC against A. fumigatus 20684.002, and AND-PSC against A. fumigatus 20684.007 at both 50% and 100% inhibition levels. The reliability of antifungal combination testing between each test event and each laboratory was evaluated by statistical analysis. Over 80% both intra- and inter- laboratory agreement rates were achieved at MIC100 reading level. The correlation coefficients of intra-laboratory agreement rates (IaCC) were greater than 0.92 with 99% confidence interval (CI) for all four laboratories. The correlation coefficients of inter-laboratory agreement rates (IeCC) were even higher (> 0.96) with 99% CI for both yeasts and molds. One hundred percent inhibition readings showed higher intra- and inter- laboratory agreement rates for two drug combinations and could be used as a basis for the development of a consensus method for routine use in the clinical laboratories.


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Full conference title: 

54th Interscience Conference of Antimicrobial Agents and Chemotherapy
    • ICAAC 54th (2014)