Antifungal Susceptibly Testing by Concentration Gradient Strip Etest Method for Fungal Isolates: A Review

Abstract

Antifungal susceptibility testing is an important tool for managing patients with invasive fungal infections, as well as for epidemiological surveillance of emerging resistance. For routine testing in clinical microbiology laboratories, ready-to-use commercial methods are more practical than homemade reference techniques. Among commercially available methods, the concentration gradient Etest strip technique is widely used. It combines an agar-based diffusion method with a dilution method that determinates a minimal inhibitory concentration (MIC) in µg/mL. Many studies have evaluated the agreement between the gradient strip method and the reference methods for both yeasts and filamentous fungi. This agreement has been variable depending on the antifungal, the species, and the incubation time. It has also been shown that the gradient strip method could be a valuable alternative for detection of emerging resistance (non-wild-type isolates) as Etest epidemiological cutoff values have been recently defined for several drug-species combinations. Furthermore, the Etest could be useful for direct antifungal susceptibility testing on blood samples and basic research studies (e.g., the evaluation of the in vitro activity of antifungal combinations). This review summarizes the available data on the performance and potential use of the gradient strip method.

Antifungal susceptibility testing practices in mycology laboratories in France, 2018

“…This variation of the incubation time (e.g. 24h vs 48h) may explain some low essential agreements between Etest and reference methods reported in the literature [9]. Therefore, new studies for determination of the optimal reading time are warranted.…”

Molecular Epidemiology of Candida Auris Outbreak in a Major Secondary-Care Hospital in Kuwait

“…Fifteen of 16 isolates from candidemia and 40 of 46 isolates from colonized patients were resistant while the remaining isolates exhibited reduced susceptibility to fluconazole at 24 h reading. Higher overall percentages of essential agreement between Etest and broth macrodilution MICs have been reported for fluconazole at 24 h compared to 48 h incubation time for clinical isolates of C. albicans, C. glabrata and C. tropicalis but not for C. parapsilosis [ 56 ]. Nearly all bloodstream isolates were also resistant to voriconazole and/or itraconazole while cross-resistance to these triazoles was lower among colonizing strains.…”

Increasing Incidence and Shifting Epidemiology of Candidemia in Greece: Results from the First Nationwide 10-Year Survey

“…In vitro susceptibility was determined as per hospital protocol using the automated susceptibility testing system Vitek ® 2 (BioMeriéux, Marcy l'Etoile, France), gradient concentration strips (Etest (BioMeriéux, Marcy l'Etoile, France), MTS (Liofilchem, Roseto degli Abruzzi, Italy)), the colorimetric assay Sensititre YeastOne (SYO; Thermo Fisher Scientific, Cleveland, OH, USA), or the broth microdilution reference method following the European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines [12,13]. Vitek ® 2 [14][15][16], Etest/MTS [17], and SYO [16,18] have been shown to give comparable results to those obtained by the Clinical and Laboratory Standards Institute (CLSI) procedure for antifungal susceptibility testing of yeasts. Thus, for the categorization of the isolates, the M60 CLSI species-specific clinical breakpoints were applied [19] and in the absence of those method-specific (CLSI (for data obtained by Vitek ® 2) [20], Etest [21][22][23], or SYO [22,24,25]), epidemiological cut-off values (ECVs) were used to differentiate wild type (WT) and non-WT phenotypes.…”