Determination of Antifungal MICs by a Rapid Susceptibility Assay

A novel microtiter assay for antifungal susceptibility testing was developed. This method has several potential advantages over the M27-A assay of the National Committee for Clinical Laboratory Standards. These include provision of MIC results within 6 to 19 h, graphical display of data, and the availability of objective quantitative endpoints. We refer to the method as the rapid susceptibility assay (RSA). RSA is based on substrate utilization by fungi in the presence of antifungal drugs. Substrate uptake is determined by a colorimetric method, which can be scored by analysis of data obtained from a microplate reader. Variables evaluated in the development of the RSA included inoculum size, incubation period, and efficacy with different classes of antifungal drugs and different yeast isolates. With the rapidly available and quantitative endpoints of the RSA, correlation of MICs and therapeutic drug doses can be evaluated more successfully than they can be evaluated by existing assays.