Comparison of Colistin Susceptibility Tests

Abstract

Introduction: Polymyxins are important antimicrobial agents for the treatment of infections caused by Gram-negative bacteria. The susceptibility testing for polymyxins is a challenge for clinical laboratories due to the difficulty of performance, reproducibility, and accuracy of available methods. Aim: To compare the performance of the colistin susceptibility test of an automated system and a gradient test with the gold standard broth microdilution method (BMD). Materials and Methods: Multidrug-resistant isolates of Acinetobacter baumannii (n=102), Klebsiella pneumoniae (n=40), and Pseudomonas aeruginosa(n=11) were included. The VITEK 2 systems and gradient test were studied according to the manufacturer’s instructions. Broth microdilution tests were performed according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST). Commercial susceptibility testing methods were compared to BMD. Results: Rates of essential agreement of colistin test results between BMD, VITEK 2, and gradient test were 96.1% and 79.7%, respectively. The VITEK 2 and gradient test showed 95.4% and 94.8% of categorial agreement. The very major error rate of VITEK 2 was 3.2%, and the gradient test was 5.2%. The major error rate of VITEK 2 was 1.3%, and there was no major error for the gradient test. Conclusion and Suggestions: The very major error rate was higher in the gradient test (5.2%) than VITEK 2 (3.2%). Even if the very major error rate of VITEK 2 was lower, both resistance and susceptility results of VITEK 2 should be confirmed with the BMD test. Further studies for susceptibility testing are needed with a focus on the correlation of MIC’s results of different tests.