Evaluation of fosfomycin susceptibility testing methods: A focus on multidrug-resistant Klebsiella pneumoniae using ECOFF values

Introduction: We aimed to evaluate the performance of two methods; disc diffusion and gradient test with the gold standard agar dilution method in determining the susceptibility of fosfomycin, in multidrug-resistant (MDR) K. pneumoniae isolates causing urinary tract infections. Methods: K. pneumoniae producing carbapenemase and extended-spectrum beta-lactamase (ESBL) isolated from urine samples submitted to the clinical microbiology laboratory were included in the study. The isolates were tested using gradient test (MTS, Liofilchem, Italy) and disc diffusion (Oxoid, UK). Agar dilution was employed as the reference method. Since there is no MIC value for K. pneumoniae in EUCAST, epidemiological cut-off values (ECOFFs) were determined and susceptibility and error rates were calculated. Results: In this study, among the 251 ESBL-positive K. pneumoniae isolates, 20(8 %) were also positive for carbapenemase. The ECOFF was determined as 128 mg/L for K. pneumoniae. When all study isolates (n:251) were considered, 87.6%(220/251) were wild-type (WT) for fosfomycin (MIC <= 128 mg/L). Among ESBL-positive but carbapenemase-negative isolates (n:231), 87.8%(203/231) were WT for fosfomycin, and among ESBL and carbapenemase-positive isolates (n:20), 85.0%(17/20) were WT. The MIC50/90 values were determined to be 8/ 256 mg/L. When compared to agar dilution, the categorical agreement was 96.8% for the gradient test and 94.8 % for disc diffusion. While the gradient test showed a 16.1% very major error (VME) rate with no major errors (ME), disk diffusion revealed 35.4 % VME rate and 5.8 % ME rate. Conclusion: A significant proportion of ESBL-positive K. pneumoniae isolates were WT for fosfomycin. The gradient test with 96.8% categorical agreement appears to be a good alternative, but agar dilution remains the gold Standard for reference laboratories.