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

Abstract : e 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 iso-lates' in vitro susceptibilities in this setting. S ince the 1990s, knowledge about the diversity of yeast species involved in human infections, the incidence of drug-resistant isolates, and antifungal drug resistance mechanisms has significantly increase (6, 10, 16, 24). In vitro susceptibility tests are based on the measurement of growth with different drug concentrations so as to determine the MIC for the population of a given isolate, an in vitro-determined value that helps predict therapeutic efficacy (1). This has been achieved with some degree of confidence by using in vivo models to determine clinical breakpoints in invasive yeast infections, providing a useful indicator to guide therapeutic choices (20). The reference tests for susceptibility testing are the broth microdilution assays devised by the Clinical and Laboratory Standards Institute (CLSI) and by the European Committee on Antibiotic Susceptibility Testing (EUCAST) (5, 23). These reference methods are robust and reproducible; however, they remain time-consuming and poorly suited for the routine clinical laboratory setting. Moreover, the MIC values for amphotericin B are tightly clustered, and these methods rarely detect MIC values above 1 mg/liter (2). To overcome these limitations, many commercially available methods, such as the Etest, Sensititre Yeast-One, or disk diffusion methods, that are easy to use in the routine setting have been developed. These methods have been recently incorporated into routine clinical laboratory practice and thus generate a considerable amount of antifungal MIC data from clinical fungal isolates. Presently, the monitoring of antifungal drug susceptibility is usually restricted to national reference laboratories that use broth microdilution assays to test clinical isolates referred from collaborating clinical laboratories. These laboratories thus collect invaluable data for the monitoring of susceptibility trends on national and international scales. However, there is a need to develop antifungal susceptibility monitoring at a local or regional scale. This complementary approach to the national reference centers could also improve patient care and generate significant cost reductions given the prevalence of yeast infections, their morbidity, and the costly protracted treatments required. As a first step toward setting up a regional survey of in vitro antifungal susceptibility in the southeast region of France, the primary aim of the present study was to assess the interlaboratory reproducibility of MICs determined with the commercially available and routinely used Etest method for yeast isolated in first-line clinical mycology laboratories of the four regional teaching hospitals. The secondary aims were to validate the correlation of the MICs of amphotericin B and caspofungin obtained with the Etest and CLSI assays at 24 h and 48 h.
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Stephane Ranque, L Lachaud, M Gari-Toussaint, A Michel-Nguyen, M Mallié, et al.. Interlaboratory Reproducibility of Etest Amphotericin B and Caspofungin Yeast Susceptibility Testing and Comparison with the CLSI Method. Journal of Clinical Microbiology, American Society for Microbiology, 2012, ⟨10.1128/JCM.00490-12⟩. ⟨hal-01310344⟩

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