Optimizing voriconazole susceptibility testing of Candida: Effects of incubation time, endpoint rule, species of Candida, and level of fluconazole susceptibility

Voriconazole is a new triazole antifungal agent that has potent activity ag ainst many isolates of Candida, including Candida krusei and Candida glabra ta. In this work, we studied the impact of glucose supplementation, incubat ion time, agitation of the plates prior to reading, endpoint determination rule, visual versus spectrophotometric reading, Candida species, and flucon azole MIC on the MIC of voriconazole for Candida isolates tested by using t he microdilution format assay of the National Committee for Clinical Labora tory Standards (NCCLS) M27-A antifungal susceptibility testing methodology. For both voriconazole and fluconazole, a spectrophotometric endpoint of 50 % reduction in turbidity relative to the growth control correlated most clo sely with the NCCLS-defined visual endpoint of "prominent decrease in turbi dity." Correlation was generally better after 24 h of incubation than after 48 h. Supplementation of the medium to contain 20 g of glucose/liter did n ot alter the MIC significantly but did enhance growth and simplify visual r eadings. All Candida species appeared potentially susceptible to voriconazo le, including isolates of C. krusei. For some isolates for which fluconazol e MICs were markedly elevated voriconazole MICs were also elevated, but the clinical significance of these observations remains to be determined.