Comparison of the E test with the CLSI Broth Microdilution Reference Method for Testing Fluconazole against Candida spp. Isolated from Blood Cultures

Abstract

Increased resistance against antifungal agents recently shows the importance of antifungal susceptibility testing especially to Candida spp. isolated from invasive infections. Although the Broth Microdilution (BMD) method has been standardized as an antifungal susceptibility testing by 'Clinical and Laboratory Standards Institute (CLSI)', difficulties in the practice of the this method pushed the researchers to seek more practical methods that can be used in routine laboratories. The aim of this study was to compare E-test method with standard BMD method for antifungal susceptibility testing of Candida spp. isolated from blood cultures in our hospital, against fluconazole which is commonly used in the treatment of systemic Candida infections. In addition by the distribution of the accumulated isolates in the process of the study, the most common species of Candida causes Candidemia in our hospital were determined.

74 Candida strains (49 C. albicans, 15 C.parapsilosis, 6 C.tropicalis, 4 C. glabrata) isolated from blood cultures (BACTEC 9000 System, Becton Dickinson ®) sent from various clinics to Ataturk University Hospital Medical Microbiology Laboratory between April 2012-December 2015 were included in the study. Chromogenic agar, automated VITEK 2 system (BioMerieux®, France) and if necessary the morphological appearance on Corn Meal Agar with Tween® 80 were used to identify Candida spp. Fluconazole susceptibility of Candida species were determined by standard BMD method defined by CLSI and E test method.

While 49 of the 74 isolated strains (66.2%) were albicans, 25 of them (33.8%) were found to be non-albicans species. The most frequently non-albicans species were C.parapsilosis (20.2%). According to the MIC values, no fluconazole resistance was interpreted for C.albicans and C.glabrata by both methods. However, 2 of 15 C.parapsilosis strains were interpreted as resistant to floconazole with both methods and 2 of 6 C.tropicalis strains were interpreted as resistant with E test method. While one of the same C.tropicalis strains was interpreted as susceptible, the other one was interpreted as susceptible-dose dependence (S-DD) with BMD method. According to these findings the total resistance rate of Candida species was 5.4% by E test method while it was determined as 2.7% in BMD method. In C.tropicalis species the result which was resistance by E test method and sensistive by BMD method was evaluated as 'major error' (ME); the result which was resistance by E test method and susceptible-dose dependence (S-DD) by BMD method was evaluated as 'minor error' (mE). Any 'very major error' (VME) was not found and the total agreement (EA) was calculated as 85.1% in the study.

According to our results that can be suggested E test method can be used as an alternative antifungal susceptibility test in routine laboratory. However, although the E test method is easy to use and has high reproducibility advantage compared with the BMD method, it requires attention and experience in the application and interpretation phases of antifungal susceptibility testing.