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

<b>Background:</b> 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. <b>Results:</b> 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 <i>P. aeruginosa</i>) than non-DTR (10% and 6.7%, resp.). All isolates of <i>S. aureus</i> 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 <i>P. aeruginosa</i>. <i>S. aureus</i> had surprisingly low essential agreement (EA) rates (53% for MRSA and 47% for MSSA) for Etest^®, but categorical agreement was 100%. <b>Methods:</b> 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 <i>Pseudomonas aeruginosa</i> (15 DTR vs. 15 non-DTR), and 60 <i>Staphylococcus aureus</i> (30 methicillin-susceptible, MSSA, vs. 30 methicillin-resistant, MRSA). <b>Conclusions:</b> 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.