Interlaboratory Reproducibility of Etest Amphotericin B and Caspofungin Yeast Susceptibility Testing and Comparison with the CLSI Method

Abstract

This study aimed to assess the interlaboratory reproducibility at four university hospital laboratories in the southeast region of France of the Etest technique for the determination of caspofungin (CAS) and amphotericin B (AMB) MICs and to compare it to the CLSI broth microdilution reference method. Consecutive clinical yeast isolates (n = 198) were included in the study. AMB and CAS MICs were read at 24 and 48 h. Interlaboratory reproducibility was estimated by using (i) an intraclass correlation coefficient (ICC), (ii) essential agreement (EA), and (iii) categorical agreement (CA). For Etest interlaboratory reproducibility for CAS, ICCs were 0.80 (95% confidence interval [CI], 0.76 to 0.84) and 0.81 (95% CI, 0.77 to 0.85) at 24 and 48 h, respectively. For AMB, the ICCs were 0.51 (95% CI, 0.43 to 0.58) and 0.69 (95% CI, 0.63 to 0.74) at 24 and 48 h, respectively. At 48 h, the between-center EAs ranged from 94.4 to 99.0% for both antifungals. For the comparison of the CLSI method and the Etest, the between-technique ICCs were 0.69 (95% CI, 0.63 to 0.74) and 0.62 (95% CI, 0.55 to 0.68) for CAS and AMB, respectively. The EAs ranged from 76.5 to 98.5% for CAS and from 90.3 to 97.4% for AMB according to the centers. CAs ranged from 87.9% to 91.4%, with four very major errors for 2 strains (1 Candida albicans strain and 1 Candida krusei strain), for CAS and from 97.5 to 99.5%, with four major errors, for AMB. In conclusion, the Etest showed a good interlaboratory reproducibility and a good correlation with the CLSI technique. It is well suited for the routine clinical laboratory and can thus be used to monitor clinical yeast isolates' in vitro susceptibilities in this setting.