Comparison of relative susceptibilities of Candida species to three antifungal agents as determined by unstandardized methods

Three groups of isolates, each comprising four isolates of Candida species, were selected for their diversity of susceptibilities to amphotericin B, flucytosine, or ketoconazole. The isolates were distributed in duplicate and in blinded fashion to three laboratories where a total of eight procedures were performed for each drug. From the decoded results, intralaboratory variability among replicate determinations was found usually to fall within a fourfold range. Interlaboratory variation, however, was 16-fold or greater for all isolates, ranging to 50,000-fold differences for some isolates. Relative susceptibilities of isolates within each method could be determined in 11 of the 24 drug-method combinations and agreed with the reference rank order in all but one instance. Our findings underscore the lack of agreement among laboratories for the susceptibility testing of yeasts but indicate that such differences could likely be resolved by standardization without loss of clinical value.