MULTISITE REPRODUCIBILITY OF THE ETEST MIC METHOD FOR ANTIFUNGAL SUSCEPTIBILITY TESTING OF YEAST ISOLATES

A multicenter study was pel formed to establish the interlaboratory re producibility of Etest, to provide an additional comparison of Etest M ICs with reference broth macrodilution MICs, and to develop some tenta tive quality control (QC) guidelines for using Etest for antifungal su sceptibility testing of Candida spp. Two QC strains, Candida parapsilo sis ATCC 22019 and Candida krusei ATCC 6258, were tested by Etest agai nst amphotericin B, fluconazole, flucytosine, itraconazole, and ketoco nazole in each of four laboratories. The QC strains were tested 20 tim es each against the five antifungal agents by using a common lot of RP MI agar. A total of 80 MICs per drug per strain were generated during the study. Overall, 98 to 100% of the MICs fell within a 3 log, diluti on range for the respective yeast-antifungal agent combinations. The l evel of agreement of Etest MICs with broth macrodilution MICs was 86 t o 100% with amphotericin B (C. krusei and C. parapsilosis), itraconazo le (C. krusei and C. parapsilosis), flucytosine (C. parapsilosis), and fluconazole (C. parapsilosis). A lower level of agreement was observe d with ketoconazole (C. krusei and C. parapsilosis), Although all part icipants reported identical Etest MICs, the MICs of flucytosine and fl uconazole when tested against C. krusei fell well above the upper limi ts of the reference range for this strain. The tentative QC limits for the two QC strains and five antifungal agents when tested by the Etes t methodology are the same as the QC limits when tested by the referen ce broth macrodilution method for amphotericin B and C. krusei, itraco nazole and C. krusei, flucytosine and C. parapsilosis, fluconazole and C. parapsilosis, and itraconazole and C. parapsilosis. The Etest QC r anges are 1 dilution broader (4-dilution range) than the reference mac rodilution method QC ranges for ketoconazole and C. krusei, amphoteric in B and C. parapsilosis, and ketoconazole and C. parapsilosis.