Evaluation of Rapid Polymyxin test in clinical isolates with various degrees of multidrug resistance.

Background: is of great concern among MDR bacteria and rapid and reliable antibiotic susceptibility testing methods are extremely necessary. Colistin is, in many cases, among the limited useful alternatives for these isolates. Unfortunately, only a few reliable methods are validated for testing susceptibility to colistin. Although EUCAST and CLSI recommend broth microdilution (BMD) as the standard method for antibiotic susceptibility testing, this method is not routinely performed in microbiology laboratories. However, some commercial products based upon BMD have tested well and offer consistent results.

Objectives: To evaluate the performance of the colorimetric Rapid Polymyxin Pseudomonas Test (RPPT) (ELITech Microbiology, France).

Methods: Eighty-seven clinical strains, prospectively collected in two microbiology laboratories exhibiting either susceptibility or various degrees of multidrug resistance, including to colistin, were used. Different susceptibility testing methods were simultaneously performed and compared with reference BMD and interpreted using 2020 EUCAST criteria.

Results: Results indicate an essential agreement (EA) of 97.7% for RPPT while the other tests did not reach 90% of EA [66.7% MicroScan, 63.2% Etest (bioMérieux, France) and 60.9% other MIC Test Strips (MTS, Liofilchem, Italy)]. The categorical agreement was 98.9% for RPPT, 87.4% for MTS, 85.1% for Etest and 64.4% for MicroScan.

Conclusions: The RPPT was able to accurately detect both colistin-susceptible and -resistant isolates within 4 h, offering a rapid alternative for a prompt decision about the inclusion of this antibiotic in a patient's treatment.