Comparison of E-test and Microbroth dilution Methods for Determining Voriconazole, Itraconazole and Amphotericin B MICs for Aspergillus fumigatus and Aspergillus flavus

Abstract

Background of the study Infections caused by filamentous fungi, especially Aspergillus fumigatus and  Aspergillus flavus are responsible for majority of infections in immunocompromised hosts. The Clinical Laboratory Standards Institute Subcommittee on Antifungal Susceptibility Tests has proposed a standard procedure for antifungal susceptibility  testing of molds (CLSI document M38-A2) . Although the  reference procedure for molds is essential for standardization of results, it is cumbersome and time-consuming other methods have been evaluated . Among these methods, Etest has been suggested as an alternative procedure for antifungal   susceptibility testing of molds. Hence the determination of invitro susceptibilities of Aspergillus isolates to the established and investigational methods are warranted .MaterialsMethods 64 isolates of clinically significant Aspergillus flavus(29) and   Aspergillus fumigatus (35) obtained from various clinical   samples were taken up for the study . Minimal inhibitory concentrations(MICs) for these isolates were determined by microbroth dilution method as per CLSI document M38-A2 and E-Test. The percentage of agreement between the E test and the reference microbroth dilution methods were   calculated. Results In this study we evaluated the in vitro          susceptibilities of 29 Aspergillus flavus isolates and 35  Aspergillus fumigatus isolates, to voriconazole, itraconazole, and amphotericin B by the E-test and CLSI M38-A2  microbrothdilution methods. An overall 100 percent agreement for voriconazole and itraconazole and 86 percent for  amphotericin B at 24 hrs and 97,93and 73 percentages  respectively at 48hrs was seen for Aspergillus flavus isolates. For Aspergillus fumigatus an overall agreement of voriconazole, itraconazole and amphotericin B was 97,91 and 97percentages respectively at 24 hrs and at 48hrs lower agreement of 71,77and 69 percentages were noted respectively. Greater discrepencies in MICs were noted at 48hours of incubation of   E-Test suggesting the importance of incubation time for the Etest. Conclusion The results of this investigation showed a good level of overall agreement between the E-test and   microbroth dilution methods . Our results suggest that the E test is suitable for routine use in susceptibility testing of Aspergillus spp. against amphotericin B and Triazoles.