Improved detection of amphotericin B-resistant isolates of Candida lusitaniae by Etest

Both intrinsic and acquired resistance to amphotericin B have been document ed for Candida lusitaniae. Amphotericin B remains the drug of choice for ma ny critical fungal infections, and the detection of resistance is essential to monitor treatment effectively. The limitations of the National Committe e for Clinical Laboratory Standards (NCCLS) reference methodology for detec tion of amphotericin B resistance are well documented, and several alternat ive methods have been proposed. Etest assays with RPMI and antibiotic mediu m 3 (AM3) agar were compared to the NCCLS M27-A broth macrodilution method using AM3 for amphotericin B resistance testing with 49 clinical isolates o f C. lusitaniae. The panel included nine isolates with known or presumed re sistance to amphotericin B on the basis of in vivo and/or in vitro data. Th e distribution of amphotericin B MICs by Etest with RPMI ranged from 0.032 to 16 mug/ml and was bimodal. All of the putatively resistant isolates were inhibited by amphotericin B at greater than or equal to0.38 mug/ml and cou ld be categorized as resistant using this breakpoint. Etest with AM3 yielde d a broader amphotericin B MIC range (0.047 to 32 mug/ml), and there were s ix putatively resistant isolates for which MICs were >1 mug/ml. The separat ion of putatively susceptible and resistant isolates was less obvious. Brot h macrodilution with AM3 generated a unimodal distribution of MICs (ranging from 0.032 to 2 mug/ml) and failed to discriminate most of the putatively resistant isolates at both 24 and 48 h. Etest using RPMI and, to a lesser e xtent, using AM3 provided better discrimination between amphotericin B-resi stant and -susceptible isolates of C. lusitaniae.