Precision and accuracy of fluconazole susceptibility testing by broth microdilution, Etest, and disk diffusion methods

Abstract

Interpretive agreements among the results of fluconazole broth microdilution tests, Etests, and disk diffusion tests were documented by evaluating 495 Candida spp. Microdilution reference test results were in agreement with 96% of the Etest results; most discrepancies were minor differences. Fluconazole resistance of Candida krusei strains often required a full 48 h of incubation in order to be observed by the standard method. For the disk diffusion tests that were performed on Mueller-Hinton agar with glucose and methylene blue, 97% of results were in agreement with those of the reference test, especially when zones of inhibition were measured after the first 24 h of incubation. Some Candida glabrata isolates failed to grow satisfactorily until a full 48 h of incubation was completed. Precision was determined by testing 50 selected isolates in triplicate in each of three laboratories. The reproducibility of results of disk diffusion tests was comparable to that of the reference method. With all procedures, determination of test results was particularly challenging with some strains, and new methods are needed in order to improve endpoint definition.

FIG. 1.

Zones of inhibition around 25-μg-fluconazole disks containing Mueller-Hinton agar supplemented with 2% glucose and methylene blue (0.5 μg/ml). Values are numbers of isolates from the 490 isolates of Candida spp. that grew on this medium after 24 h. The least-squares method was used to calculate a regression line (y = 55.4 − 2.66x, with r = 80).