Study On The Evaluation Of The Methods For Colistin Susceptibility Testing In Vitro And The Heteroresistance Of Carbapenem-resistant Acinetobacter Baumannii

Objective:The current study aimed at evaluating the 4 available methods to test in vitro susceptibility to colistin and observing the degree of heteroresistance in CRAB species in China.Methods:To evaluate the methods,a total of 202 CRAB species isolated from 12 hospitals in Zhejiang province,China,collected from January to December in 2010 were employed retrospectively.Colistin minimum inhibitory concentrations(MICs)were determined by the 4 different methods.Population analysis profiles(PAPs)were also conducted in 29 CRAB strains.Results:The proportions of colistin-sensitive isolates were 100%,100%,99.5%,and 90.6%by BMD-P80,E-test,BMD,and AD methods,respectively,according to the EUCAST breakpoints.The AD methods produced an excessive number of major errors(MEs)(9.4%),while E-test and BMD produced 0.5%MEs.Moreover,the AD method resulted in a minimum essential agreement(EA)at 9.4%,and 179 isolates obtained a higher(≥3 log 2)number of dilutions than the BMD-P80.Not very major errors(VMEs)were found by any of the tested methods.In 29 selected CRAB isolates,a total of 65%were heterogeneous and the rate of heteroresistance was 31%.Conclusion:The AD method was not a prior choice of in vitro colistin susceptibility testing because it led to false colistin resistance results.E-test,BMD,and BMD-P80 may be more reliable to test the susceptibility when colistin is considered as a potential therapeutic agent.A high rate of heterogeneous and heteroresistant species were found in CRAB in China;it highlighted the importance of MICs monitoring before and through the colistin therapy period.