Comparison of Three Different Commercial Methods for Fosfomycin Susceptibility Testing in Pseudomonas aeruginosa
Marco Peradotto1, Alessandro Bondi1, Gabriele Bianco1
1Microbiology and Virology Unit, Città della Salute e della Scienza di Torino, University of Turin, Torino, Italy.
Abstract:
The lack of internationally approved breakpoint and a handy susceptibility testing reduces fosfomycin usefulness against Pseudomonas aeruginosa. Previous works defined low or moderate agreement between commercial methods and the reference method (agar dilution [AD]). In particular, data lack about testing against P. aeruginosa. We compared disk diffusion (DD), E-test (ET), and automated broth microdilution (BMD) to AD by testing 150 P. aeruginosa isolates. We obtained better categorical agreement (CA) for DD and ET for minimal inhibitory concentration >128 mg/L (84.7% and 92.7%, respectively), but with high very major error (VME). BMD had the lowest VME rate (2/42), but with 64% CA and 52/108 major errors. We cannot define a method comparable to AD. Larger studies, as well as the definition of a breakpoint value are needed for fosfomycin against P. aeruginosa.
Insights
Accurate susceptibility testing for fosfomycin against Pseudomonas aeruginosa is lacking. Current commercial methods show poor agreement with the reference method, hindering effective treatment decisions for this pathogen.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Infectious Diseases
Background:
- Fosfomycin's clinical utility against Pseudomonas aeruginosa is limited by the absence of standardized susceptibility testing methods and approved breakpoints.
- Previous studies indicated poor agreement between commercial susceptibility tests and the reference agar dilution (AD) method for P. aeruginosa.
Purpose of the Study:
- To evaluate the performance of disk diffusion (DD), E-test (ET), and automated broth microdilution (BMD) against the reference agar dilution (AD) method for testing fosfomycin susceptibility in P. aeruginosa isolates.
- To assess the categorical agreement (CA) and error rates of commercial methods compared to AD for P. aeruginosa.
Main Methods:
- 150 P. aeruginosa isolates were tested using disk diffusion (DD), E-test (ET), automated broth microdilution (BMD), and agar dilution (AD) as the reference method.
- Categorical agreement (CA) and very major error (VME) rates were calculated for each commercial method against AD.
Main Results:
- Disk diffusion (DD) and E-test (ET) showed higher CA for minimal inhibitory concentration >128 mg/L (84.7% and 92.7%), but with significant very major error rates.
- Automated broth microdilution (BMD) exhibited the lowest VME rate (2/42), but achieved only 64% CA and had a high number of major errors (52/108).
- No commercial method demonstrated sufficient agreement with the reference AD method for reliable fosfomycin susceptibility testing in P. aeruginosa.
Conclusions:
- Current commercial methods for fosfomycin susceptibility testing against P. aeruginosa are not comparable to the reference agar dilution method.
- Larger-scale studies are required to validate alternative methods.
- The establishment of an internationally approved breakpoint value for fosfomycin against P. aeruginosa is crucial for clinical application.
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