Comparison of Caspofungin MICs by Means of EUCAST Method EDef 7.1 Using Two Different Concentrations of Glucose

According to the product insert for Cancidas (caspofungin acetate), the drug must not be diluted in solutions containing glucose as this decreases caspofungin stability. The aim of this study was to compare caspofungin MICs for a collection of yeasts by means of EUCAST method EDef7.1 but using two different concentrations of glucose: 2% versus 0.2%. MICs were identical or within one 2-fold dilution for 93 out of 95 strains (97.9%), showing that glucose does not interfere with susceptibility.In a previous work, a comparison of the major susceptibility testing methods for echinocandins was performed using a well-characterized panel of Candida strains (1). EUCAST method EDef 7.1 performed as one of the best methods for discriminating wild-type strains from isolates with mutations in FKS hot spot regions. However, for caspofungin the MICs obtained by EUCAST method EDef 7.1 were in general higher than those obtained using the CLSI M27A3 methodology. In addition, the number of very major errors (number of FSK hot spot mutants classified as susceptible according the wild-type upper limit values) was 50% for EUCAST method EDef 7.1 (1), while 7% of very major errors were obtained by means of CLSI M27A3 at 24 h (1, 2). Finally, this previous work also found that caspofungin MICs obtained by means of EUCAST method EDef 7.1 and CLSI M27A3 were, in general, higher than those obtained by Pfaller et al. in a previous work using CLSI M27A3 methodology (4), indicating a possible variability associated with caspofungin antifungal susceptibility testing (1). Thus, it was of interest to explore which variables influence the reproducibility of the susceptibility tests for caspofungin. Although it has been proven that both the EUCAST EDef 7.1 and CLSI M27 A3 (2, 5) methodologies generate similar results for amphotericin B and azole drugs (3, 6), this does not appear to be the case for caspofungin. The main differences between EUCAST method EDef 7.1 and CLSI M27A3 include different inocula (10⁵ versus 10³ CFU/ml, respectively) and glucose concentrations (2% versus 0.2%, respectively) (3, 6).The product insert for Cancidas states that the compound must not be diluted in solutions containing glucose because it decreases drug stability. As stated previously, the glucose concentration in the growth medium recommended for method EDef 7.1 in RPMI 1640 is 10 times higher than that recommended by the CLSI (5). As this concentration could have influenced the MIC values obtained by means of EUCAST method EDef 7.1 (5), the Antifungal Susceptibility Testing Subcommittee of EUCAST decided to set up a comparison test to examine whether the increased glucose concentration affects the MIC results. The test compared caspofungin MICs for a collection of well-characterized yeast isolates by means of EUCAST method EDef 7.1 (5) but using growth medium with two different concentrations of glucose: 2% versus 0.2%.