Interlaboratory comparison of results of susceptibility testing with caspofungin against Candida and Aspergillus species

Abstract

Seventeen laboratories participated in a study of interlaboratory reproducibility with caspofungin microdilution susceptibility testing against panels comprising 30 isolates of Candida spp. and 20 isolates of Aspergillus spp. The laboratories used materials supplied from a single source to determine the influence of growth medium (RPMI 1640 with or without glucose additions and antibiotic medium 3 [AM3]), the same incubation times (24 h and 48 h), and the same end point definition (partial or complete inhibition of growth) for the MIC of caspofungin. All tests were run in duplicate, and end points were determined both spectrophotometrically and visually. The results from almost all of the laboratories for quality control and reference Candida and Aspergillus isolates tested with fluconazole and itraconazole matched the NCCLS published values. However, considerable interlaboratory variability was seen in the results of the caspofungin tests. For Candida spp. the most consistent MIC data were generated with visual "prominent growth reduction" (MIC(2)) end points measured at 24 h in RPMI 1640, where 73.3% of results for the 30 isolates tested fell within a mode +/- one dilution range across all 17 laboratories. MIC(2) at 24 h in RPMI 1640 or AM3 also gave the best interlaboratory separation of Candida isolates of known high and low susceptibility to caspofungin. Reproducibility of MIC data was problematic for caspofungin tests with Aspergillus spp. under all conditions, but the minimal effective concentration end point, defined as the lowest caspofungin concentration yielding conspicuously aberrant hyphal growth, gave excellent reproducibility for data from 14 of the 17 participating laboratories.

FIG. 1.

Dose-response curves for caspofungin growth inhibition constructed from individual spectrophotometric data presented by 16 laboratories. Curves show growth of a C. albicans isolate as mean percent control in the presence of a caspofungin concentration series, with error bars showing the standard error of the mean (n = 32). Cultures were in RPMI (○), RP-G inoculated to 10³/ml (□), RP-G inoculated to 10⁵/ml (▴), and AM3 (•). (a) Y102 after 24 h of incubation; (b) Y106 after 48 h of incubation.