Multicentre validation of 4-well azole agar plates as a screening method for detection of clinically relevant azole-resistant Aspergillus fumigatus

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Author: 

Maiken Cavling Arendrup Paul E Verweij Johan W Mouton Katrien Lagrou Joseph Meletiadis
Journal of Antimicrobial Chemotherapy, dkx319, 04/10/2017

Abstract: 

Objectives: Azole-resistant Aspergillus fumigatus is emerging worldwide. Reference susceptibility testing methods are technically demanding and no validated commercial susceptibility tests for moulds currently exist. In this multicentre study a 4-well azole-containing screening agar method was evaluated using clinically relevant isolates.
Methods: Forty WT and 39 cyp51A mutant A. fumigatus [G54 (n =10), M220 (n =10), TR34/L98H (n =9) and TR46/Y121F/T289A (n =10)] were tested individually and as simulated mixed samples (sampling 4 WT and 1 mutant colonies). EUCAST MICs were determined following E.Def 9.3. In-house and commercial 4-well plates containing agars supplemented with 4 mg/L itraconazole, 1 mg/L voriconazole, 0.5 mg/L posaconazole and no antifungal, respectively, were evaluated. Growth was scored (0–3) by two independent observers in three laboratories. Inter-plate, inter-observer, essential and categorical agreement, sensitivity and specificity were calculated.
Results: CYP51A genotype and antifungal compound-specific MICs and growth patterns were documented. The inter-observer agreement was excellent with 86%–99% identical scores (range 80%–100%) for both plates. The qualitative agreement (no growth versus growth) was excellent (median 95%–100%, range 87%–100%, overall). The overall sensitivity and specificity for the 4-well plate (no growth versus growth) was 99% (range 97%–100%) and 99% (95%–100%), respectively. The sensitivity for simulated WT/mutant specimens was 94% (range 83%–100%) for the WT-TR34/L98H combination, but 100% for the WT/G54W combination. The performance remained unchanged using only itraconazole- and voriconazole-containing agars, but was lower for the other combinations.
Conclusions: Implementation of the 4-well screening plate in routine laboratories will allow easy and reliable detection of the most common azole-resistant A. fumigatus.