Background: EUCAST 9.3.1 procedure recommends visual MIC reading (complete fungal growth inhibition) with azoles and amphotericin B against Aspergillus spp. As visual MIC setting may be challenging, we obtained spectrophotometric MICs readings of azoles and amphotericin B against A. fumigatus complex isolates and compared them with visual method to overcome subjectivity.
Materials/methods: A total of 848 A. fumigatus complex clinical isolates collected in a 30-hospital survey conducted in Spain were studied. A. fumigatus sensu stricto isolates included 45 azole-resistant isolates with the following cyp51A gene mutations: TR34-L98H (n=24), G54R (n=5), TR46/Y121F/T289A (n=1), F46Y/M172V/N248T/D255E/E427K (n=2), F46Y/M172V/ N248T/D255E/E416Q/E427K (n=1), F165L (n=1), S496L (n=1), and wild type (n=10). Antifungal susceptibility to amphotericin B, itraconazole, voriconazole, posaconazole, and isavuconazole was performed according to EUCAST 9.3.1 methodology. Visually-set MICs were compared with spectrophotometrically-obtained MICs (fungal growth reduction >95% compared to control and read at 540 nm); essential (±1 twofold dilution) and categorical agreement were calculated. Errors were classified as very major (isolate classified as resistant by visual MIC and as susceptible by spectrophotometric reading) and major (isolate classified as susceptible by visual MIC and as resistant by spectrophotometric reading).
Results: Overall, essential agreement was 97%, with amphotericin B and posaconazole showing the highest agreement (Table). Categorical agreement was very high (98%) as well, with a total of 1.8% very major errors found in six A. fumigatus sensu stricto isolates that were resistant to either voriconazole and isavuconazole [n=3] or to isavuconazole [n=3], and four cryptic species (N. udagawae [n=2], A. fumigatiaffinis [n=2]). No very major errors were found with amphotericin B or itraconazole. Major errors were scarce and exclusively found with voriconazole and posaconazole against A. fumigatus sensu stricto (<1%; n=5 isolates). All cyp51 gene mutants were correctly classified as resistant. Most of errors occurred in MICs just one two-fold dilution above the breakpoint.
Conclusions: MICs of azoles and amphotericin B against A. fumigatus obtained either by spectrophotometer or visually showed very high agreement. Cyp51A mutants were correctly classified as resistant and most misclassifications occurred in MICs just one two-fold dilution above the breakpoint.
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Full conference title:
- ECCMID 30th (2020)