Evaluation of In Vitro Activity of Ceftazidime-Avibactam and Colistin with VITEK 2 AST-XN21

Ceftazidime-avibactam and colistin are antibiotics of new and regaining importance used for the treatment of infections caused by multidrug-resistant organisms. The broth microdilution (BMD) test recommended for detecting colistin sensitivity is labor-intensive and difficult to perform under routine conditions. There is a need for alternative methods that produce fast and reliable results in routine laboratory studies. In our study, we aimed to compare the results obtained with the newly introduced Vitek-2 AST-XN21 cards, which allow the detection of ceftazidime-avibactam and colistin sensitivity simultaneously, with the ceftazidime-avibactam disk diffusion and Diagnostics MIC COL results that we use routinely.

A total of 60 isolates, including 51 carbapenem-resistant Klebsiella pneumoniae and three each of Echerichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii isolated from various clinical samples, were included in the study. Identification of strains and antibiotic susceptibility tests were performed with MALDI-TOF MS and Vitek-2 (bioMerieux, France) system. Ceftazidime-avibactam sensitivity was determined by disk (10 - 4 µg) diffusion method. Colistin minimum inhibitory concentration (MIC) was studied with the Diagnostics MIC COL kit at a dilution range of 0.25 - 16 mg/L. Susceptibility results were evaluated according to EUCAST V.14.0 recommendations. The performance of the Vitek-2 AST-XN21 kit was evaluated according to ISO criteria.

By disk diffusion method, 31 of 57 isolates (54.4%) were found resistant to ceftazidime-avibactam. With the Diagnostics MIC COL kit, 31 of 60 isolates (51.7%) were detected as colistin resistant. Ceftazidime-avibactam and colistin sensitivity results determined by the AST-XN21 card were found to be categorically compatible with the ceftazidime-avibactam disk diffusion and colistin BMD results (100%); no major or very major errors were detected. Since the colistin BMD MIC reading limits of the Diagnostics MIC COL and AST-XN21 tests are different (16 mg/L; 8 mg/L), the essential agreement for isolates (n = 31) with MIC values < 0.25 mg/L and > 8 mg/L were not compared. The essential agreement for the evaluated 29 isolates was found to be 96.5%.

Our results showed that the performance of the Vitek-2 AST-XN21 test was good. According to our data, we believe that the Vitek-2 AST-XN21 test can be used in routine laboratories for the detection of ceftazidime-avibactam and colistin resistance.