Evaluation of a breakpoint test for determination of fluconazole susceptibility of yeasts

Abstract

Using the breakpoint test at 1 g/ml and 4 g/ml fluconazole, a minimal inhibitory concentration (MIC) of < or = 4 g/ml fluconazole was determined against 78.5% of the 1254 clinical yeast isolates. When compared with the micro broth dilution test, none of a subset of 128/1254 strains had a higher MIC in the dilution test than in the breakpoint test, however, in 43.0% of the 128 strains the MIC was lower in the micro broth dilution test when compared to the MIC of the breakpoint test. In a subset of 94 strains with an MIC of > 4 g/ml fluconazole determined in the breakpoint test, the elevated MIC could be confirmed only in 45.7% of the strains when using the micro broth dilution test. The percentage of breakpoint test confirmation as well as the number of strains with decreased susceptibility towards fluconazole (> 4 g/ml) were species dependent, thus, the number of decreased-susceptible Candida albicans strains was smaller than that of C. glabrata or other Candida species such as C. krusei, C. inconspicua and some C. tropicalis strains. The breakpoint test allows to identify susceptible strains with a high accuracy. Strains with an MIC > 4 g/ml fluconazole should be tested in the micro dilution test to confirm decreased susceptibility and thus to indicate the need for higher dosage of fluconazole or a change of the antifungal therapy. The breakpoint test proved to be a rapid and reliable screening test.