Factors influencing susceptibility testing of antifungal drugs: a critical review of document M27-A4 from the Clinical and Laboratory Standards Institute (CLSI)

Due to the increasing numbers of fungal infections and the emergence of drug-resistant fungi, optimization and standardization of diagnostic methods for the measurement of antifungal susceptibility are ongoing. The M27-A4 document by the US Clinical and Laboratory Standards Institute (CLSI) is presently used for the interpretation of minimum inhibitory concentrations of major opportunistic yeast species as measured by broth microdilution testing in many countries. Although microdilution is considered a benchmark for reproducible and accurate results, increased testing capacity, and limited human bias, the method is often inaccessible to routine clinical laboratories and researchers, especially in low-income countries. Furthermore, several studies suggest that there are still a considerable number of factors that make the estimation of in vitro activity of antifungal agents challenging. This review article summarizes the limitations of the M27-A4 standard which, despite the advances and improvements obtained by the standardization of antimicrobial resistance testing methods by CLSI, still persist.