Evaluation of imipenem-relebactam susceptibility testing in carbapenem resistant Pseudomonas aeruginosa: comparison of sensititre microplates, disc diffusion, and MTS gradient strips with broth microdilution.

Four commercial tests EUMDROXF® Sensititre microplates, MTS gradient strips, and disc diffusion were evaluated for imipenem-relebactam susceptibility in 148 ceftazidime and imipenem-resistant Pseudomonas aeruginosa strains, using broth microdilution (BMD) as the reference. EUMDROXF® Sensititre microplates showed the highest accuracy (CA = 93.2%, EA = 93.9%, and bias difference + 21.8%). While the MTS gradient strips showed acceptable performance (CA = 84.5%; EA = 89.9%, and difference of bias = 21.0%), the disc method misclassified 25.8% (16/62) of the resistant strains. Given these results, considering an Area of Technical Uncertainty (ATU) for disc diffusion (20-24 mm) in imipenem-relebactam testing is recommended for P. aeruginosa strains.