Influences of methodological variables on susceptibility testing of caspofungin against Candida species and Aspergillus fumigatus

The influences of test variables on the outcome of susceptibility testing with caspofungin were tested with isolates of Candida spp. and Aspergillus fumigatus. Among six growth conditions tested with a range of inoculum sizes, the highest control growth yields were obtained in Sabouraud broth for all fungi, followed by RPMI 1640 (pH 7) for Candida spp. and antibiotic medium 3 (AM3) for A. fumigatus. RPMI 1640 gave unacceptably low growth yields with A. fumigatus. The caspofungin MICs under these various conditions ranged over more than 4 twofold dilutions for 7 of 16 fungi tested when a 50% inhibition (50% inhibitory concentration [IC50]) endpoint was used and for 12 of 16 fungi tested when an 80% inhibition (IC80) endpoint was used. A multifactorial design to study the influences of six test variables on control growth and the MIC showed that, for 14 isolates of Candida spp., the glucose concentration and the medium composition were the most common factors significantly influencing both control growth yields and the MIC. For eight A. fumigatus isolates, incubation time (24 versus 48 h) and temperature (30 versus 35degreesC) significantly affected control optical density (OD) values, while growth medium (AM3 versus Sabouraud broth) was the most common process variable affecting the MICs. Tests with AM3 from three suppliers showed significant variations in control OD values related to supplier, but IC(50)s fell within a 2- or 3-dilution range for 19 (86%) of the 22 isolates tested. We recommend that, at present, AM3 is superior to RPMI 1640 for testing of the susceptibilities of both yeasts and filamentous fungi to caspofungin and that a minimum incubation time of 48 h is necessary to test A. fumigatus adequately.