Wild-type MIC distributions and epidemiologic cutoff values for fluconazole, posaconazole, and voriconazole when testing Cryptococcus neoformans as determined by the CLSI broth microdilution method

Abstract

When clinical susceptibility breakpoints (CBPs) are absent, establishing wild-type (WT) MIC distributions and epidemiologic cutoff values (ECVs) provides a sensitive means for detecting emerging resistance to antimicrobials. We determined species-specific ECVs for fluconazole (FLC), posaconazole (PSC), and voriconazole (VRC) using a large global collection of Cryptococcus neoformans (CNEO) isolates obtained from the ARTEMIS and SENTRY Antimicrobial Surveillance Programs. From 2006 to 2009, 285 invasive clinical isolates of CNEO were collected from 61 centers worldwide (178 isolates from ARTEMIS and 107 from SENTRY) and susceptibility testing was performed against FLC, PSC, and VRC using Clinical and Laboratory Standards Institute M27-A3 broth microdilution method (72 h of incubation). The ARTEMIS isolates were tested at the University of Iowa and the SENTRY Program isolates were tested at JMI Laboratories, and the results were combined for analysis. An additional collection of 986 isolates tested against FLC between 1996 and 2008 were used to assess temporal trends in the frequency of non-WT isolates. The modal MICs (mg/L) for FLC, PSC, and VRC were 4, 0.12, and 0.06, respectively. The ECVs expressed as milligrams per liter (% of isolates that had MIC ≤ECV) for FLC, PSC, and VRC were 8 (96.9), 0.25 (96.5), and 0.12 (95.1), respectively. Temporal trends in the emergence of non-WT strains (% of isolate MICs >ECV) for the time periods 1996–2000, 2001–2004, and 2005–2008 for FLC were 4.2, 3.8, and 0.5, respectively. In the absence of CBPs for FLC, PSC, and VRC, these WT MIC distributions and ECVs will be useful in surveillance for detection of emergence of azole reduced susceptibility among CNEO. Application of the FLC ECV to a large collection of CNEO tested over time (1996–2008) revealed a decrease in the frequency of non-WT strains. These findings are consistent with those of more limited surveys in developed countries, suggesting that CNEO susceptibility to FLC has improved since the introduction of antiretroviral therapy. Continued surveillance using these ECVs for the azoles and CNEO appears warranted.