Fosfomycin-Overcoming Problematic In Vitro Susceptibility Testing and Tricky Result Interpretation: Comparison of Three Fosfomycin Susceptibility Testing Methods.

Fosfomycin (FOS) is an older antimicrobial agent newly rediscovered as a possible treatment for infections with limited therapeutic options (e.g., Gram-negative bacteria with difficult-to-treat resistance, DTR), especially in intravenous form. However, for correct usage of FOS, it is necessary to have a reliable susceptibility testing method suitable for routine practice and robust interpretation criteria. The results were interpreted according to 2023 interpretation criteria provided by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). DTR Gram-negatives were more likely to be resistant to FOS (45% in Enterobacterales and 20% in ) than non-DTR (10% and 6.7%, resp.). All isolates of were susceptible to FOS. In Gram-negatives, all agreement values were unacceptable. Etest performed better in the DTR cohort (categorical agreement, CA, 80%) than in the non-DTR cohort (CA 45.7%). There were no very major errors (VREs) observed in . had surprisingly low essential agreement (EA) rates (53% for MRSA and 47% for MSSA) for Etest, but categorical agreement was 100%. A total of 130 bacterial isolates were tested and compared using the disc diffusion method (DD) and gradient strip method (Etest) with the reference method (agar dilution, AD). The spectrum of isolates tested was as follows: 40 Enterobacterales (20 DTR vs. 20 non-DTR), 30 (15 DTR vs. 15 non-DTR), and 60 (30 methicillin-susceptible, MSSA, vs. 30 methicillin-resistant, MRSA). Neither one of the tested methods was identified as a suitable alternative to AD. It would be beneficial to define more interpretation criteria, at least in some instances.