Comparison of a new colorimetric assay with the NCCLS broth microdilution method (M-27A) for antifungal drug MIC determination

We evaluated a new microtiter assay for antifungal susceptibility testing b ased on a colorimetric reaction to monitor fungal substrate utilization. Th is new method (rapid susceptibility assay [RSA]) provides quantitative endp oint readings in less than 8 h compared with visual determination of MIC by the National Committee for Clinical Laboratory Standards (NCCLS) broth mic rodilution method, which requires a minimum of 48 h of incubation, In this study, we tested clinical isolates from each of the following species: Cand ida albicans (20 isolates), C. glabrata (20 isolates), C. krusei (19 isolat es), C. tropicalis (19 isolates), and C. parapsilosis (28 isolates). RSA an d NCCLS broth dilution methods were used to determine the MICs of amphoteri cin B, fluconazole, itraconazole, and 5-flucytosine for all 106 isolates. R PMI 1640 medium buffered with morpholinopropanesulfonic acid was used for b oth methods; however, glucose and inoculum concentrations in the RSA were m odified. RSA MICs were determined as the lowest drug concentration that pre vented glucose consumption by the organism after 6 h of incubation. MICs ob tained from the RSA were compared with those obtained from the NCCLS M-27A method read at 24 and 48 h. MIC pairs were considered in agreement when the difference between the pairs was within 2 twofold dilutions. For the 106 i solates tested, amphotericin B and 5-flucytosine demonstrated the highest a greement in MICs between the two methods (100 and 98%, respectively), where as fluconazole and itraconazole produced less favorable MIC agreement (63.2 and 61.3%, respectively). The azole MIC differences between the two method s were significantly reduced when lower inocula were used with a prolonged incubation time. This preliminary comparison suggests that this rapid proce dure may be a reliable tool for the in vitro determination of MICs of ampho tericin B and 5-flucytosine and warrants further evaluation.