Agar-based screening of azole resistance in Aspergillus fumigatus

Jochem B Buil


Background: Azole resistance in Aspergillus fumigatus is a   global emerging health problem. Antifungal susceptibility testing is a valuable tool for guiding therapy, although EUCAST and CLSIreference methods are mostly only available in specialized centers. We studied the performance of anagar-based screenings method, VIPcheck™, developed at our center for the detection of azole resistance in A. fumigatuscultures. 

Material/methods: The VIPcheck™ consists of four wells containing agar with either voriconazole, itraconazole or posaconazole. The fourth well is intended as growth control. Ninety-six A. fumigatusisolateswere inoculated on the VIPcheck™ in duplicate. Thirty-three isolates harbored a known resistance mechanism: TR34/L98H (11 isolates); TR46/Y121F/T289A (6 isolates); TR53(2 isolates); Fourteen isolates with Cyp51A-gene point mutations conferring azole resistance. For eighteenisolates, noCyp51A-mediated azole resistance mechanism was found. Four technicians (observer 1-4) and two inexperienced interns (observer 5-6), read the plates visually after 24 h and 48 h and documented minimal, distinct or no growth for each individual well. The outcome was compared to the EUCAST reference method.

Results: The mean positive predictive value (ppv) for distinct growth after 24 hours for all 6 observers was 1.00, with a  mean negative predictive value (npv) of 0.53. The results after 48 hours with minimal growth as threshold for azole resistance are displayed at Table 1.

Conclusions: The VIPcheck™ proved to be an easy-to-apply and reliable method for azole resistancedetection of A. fumigatus isolates. The results indicate that distinct fungal growth on an azole containing well after 24 hours could be reliably interpreted as indication for azole resistance. Absence of growth after 24 hours however could not exclude azole resistance as the mean npv was only 0.53. After 48 hours, the mean ppv and npv were 0.98/0.97 respectively for the four technicians as minimal growth was used as threshold. These results indicate that the VIPcheck™ might be used as a dependable tool for azole resistance screening and it should be evaluated further in a   multicenter setting.



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Full conference title: 

European Congress of Clinical Microbiology and Infectious Diseases
    • ECCMID 26th (2016)