Multilaboratory Testing of Antifungal Drug Combinations against Candida Species and Aspergillus fumigatus: Utility of 100 Percent Inhibition as the Endpoint

Four laboratories tested three isolates of Candida species and two isolates of Aspergillus fumigatus using 96-well plates containing combinations of amphotericin B, anidulafungin, caspofungin, micafungin, fluconazole, itraconazole, posaconazole, and voriconazole. The majority of summation fractional inhibitory concentration indices (Sigma FICI) based on the Lowe additivity formula suggested indifferent drug interactions (Sigma FICI > 0.5 and <= 4.0) and no instance of drug antagonism (Sigma FICI > 4.0). The intra-and interlaboratory agreement rates were superior when MIC100 readings were used as endpoints (at a 99% confidence interval [CI]).