Scope and frequency of fluconazole trailing assessed using EUCAST in invasive Candida spp. isolates

Trailing is a well-known phenomenon that is defined as reduced but persistent visible growth of Candida spp. at fluconazole concentrations above the MIC. Trailing is commonly detected using the CLSI M27-A3 method, although little is known about its frequency when investigated with EUCAST. We assessed the frequency and scope of fluconazole trailing after using EUCAST EDef 7.2. against a large number of Candida spp. isolates from patients with candidemia. We studied 639 fluconazole-susceptible non-krusei Candida spp. isolates from 570 patients admitted to Gregorio Maranon Hospital. Isolates were tested in vitro for fluconazole susceptibility according to the EUCAST EDef 7.2 procedure; trailing was defined as the presence of any residual growth in wells containing fluconazole concentrations above the MIC. According to the mean percentage of trailing observed, isolates were classified as residual trailers (0.1-5%), slight trailers (6%-10%), moderate trailers (11%-15%), and heavy trailers (>15%). The relationship between trailing and genotyping was assessed. The mean overall percentage of trailing was 6.8%, with C. albicans and C. tropicalis showing the highest percentages (9.75% and 9.29%, respectively; P < .001). C. albicans and C. tropicalis had the highest percentage of heavy trailers (>15%). Trailing was not genotype-specific. Fluconazole trailing was observed frequently when EUCAST was used for antifungal susceptibility testing, particularly in isolates of C. albicans and C. tropicalis. The cut-off proposed enabled us to classify the isolates according to the degree of trailing and can be used as the basis for future studies to evaluate the clinical impact of this phenomenon.