Correlation between Etest (ET), Disk diffusion (DD), and Microdilution (MD) Antifungal Susceptibility Testing (AFST) of Fluconazole (FLU) and Voriconazole (VOR).



Background: ET and DD AFST methods are technically less challenging. Correlations with the NCCLS M27-A MD method are required. Methods: 400 bloodstream isolates of Candida spp. were tested against FLU & VOR by ET, DD, & MD (205 C. albicans, 56 C. tropicalis, 39 C. glabrata, 66 C. parapsilosis, 24 C. krusei, and 10 other spp.). MD followed NCCLS M27-A. DD (25 µg FLU & 1 µg VOR, by Becton Dickinson and Remel, respectively) & ET (AB Biodisk, Sweden), were done on Mueller-Hinton (MH) agar with 2% glucose, flooded with 0.5 µg/mL methylene blue (MB, Diagn Microbiol Infect Dis 2000; 36:215). MICs and zones were read at prominent growth inhibition. FLU MICs by MD and ET were interpreted by M27-A criteria. FLU zones by DD were interpreted as S ( 19mm), S-DD (15-18mm), R ( 14mm) per Barry (J Clin Microbiol 34:2154, 1996). DD zones and log-transformed MD & ET MICs were compared by linear correlation. Results: Good-to-excellent correlations were noted overall, with Pearson r 0.7 (FLU) and 0.5 (VOR) for 48h NCCLS-standard MD vs. both 24h & 48h ET and DD. 24h MD readings were correlated better with ET/DD (r 0.8 and 0.6 for FLU & VOR, respectively). ET and DD consistently correlated well (r 0.8). For FLU, correlation based on interpretive category is also possible: ET & DD yielded the same interpretation as MD at 48h for 96% of isolates. Inconsistent readings were seen with trailing growth: isolates were usually susceptible by ET and DD (24h & 48h), but resistant at 48h/susceptible at 24h by MD. Conclusion: DD and ET on MB-MH agar correlated well with 48h MD reference method. The 24h MD reading produced slightly better correlations, especially for isolates with trailing growth. Such isolates have been shown susceptible in vivo (AAC 1998;42:129 & 2000;44:2081). The demonstration of susceptibility by DD/ET but resistance by MD suggests an advantage to these agar-based methods.

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Full conference title: 

42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
    • ICAAC 42nd