ISO standard 20776-1 or serial 2-fold dilution for antifungal susceptibility plate preparation: that is the question

Background Since the ISO standard 20776-1 and serial dilution procedures were compared in 2010 for fluconazole and itraconazole, several new antifungals that are hydrophobic and highly potent have been introduced. Objectives To investigate the impact of the number of tip changes during serial dilution, and ISO and serial dilution for nine antifungals. Methods EUCAST E.Def 7.3.2 with serial (0–10 tip changes) and ISO dilution. Candida parapsilosis ATCC 22019, Candida albicans ATCC 64548, C. albicans CNM CL-F8555, Candida krusei ATCC 6258, Aspergillus flavus ATCC 204304 and clinical isolates (n = 5) of C. albicans, Candida dubliniensis, Candida glabrata, C. krusei, A. flavus and Aspergillus terreus were included. GM MICs were compared for ISO and serial dilution and with QC values where available. Results Increasing the number of tip changes (0/1/2/10 times) during serial dilution for plate preparation increased the MICs 1 to >2 dilutions for amphotericin B, anidulafungin, micafungin, fluconazole, voriconazole and isavuconazole against C. albicans ATCC 64548 but only isavuconazole MICs against C. parapsilosis ATCC 22019 (3 dilutions). ISO and serial dilution (two tip changes) were compared for eight compounds and four Candida QC strains (352 MICs). Six/41 GM MIC pairs deviated with 1–1.8 dilution (14.6%). Comparing the GM MIC with the QC values, the ISO method GM MIC was closest to the target in 30.8%, the serial dilution in 34.6% and the methods identical in 34.6% of the cases. Finally, ISO and serial dilution MICs were compared for clinical isolates (920 MICs). Five/23 GM MIC pairs (21.7%) deviated 1.0–1.1 dilutions. Conclusions The ISO and serial dilution (two tip changes) method were in acceptable agreement and thus equally applicable for EUCAST testing.