Evaluation of the “Dip Effect” Phenomenon in Antifungal Susceptibility Testing of Candida spp. against Echinocandins by Use of Gradient Concentration Strips

The "dip effect" phenomenon complicates antifungal susceptibility testing with gradient concentration strips. Of 60 Candida isolates tested with the three echinocandins, this phenomenon was observed only for caspofungin with most (>90%) Candida albicans, Candida glabrata, and Candida tropicalis isolates and for isolates with CLSI MICs of ≤0.25 mg/liter. In order to facilitate MIC determination, a practical approach was developed using the inhibition zones at 32, 8, 2, and 1 mg/liter, increasing the agreement with the CLSI method >86%.