Reliability of MIC Gradient Strips (E-test) in Detection of Colistin Resistant Acinetobacter baumannii Caused an Outbreak in a Teaching Hospital in Tehran

Colistin resistant Acinetobacter baumannii strains have become an important treat in nosocomial infection control. The reliable detection of these strains plays a critical role in treatment procures. The aim of this study was to evaluate the three different methods in detection of colistin resistant A. baumannii strains. Eighty-three A. baumannii strains were isolated from hospitalized patients of a teaching hospital in Tehran during 1 year (2016-2017). All isolates were genetically confirmed by Polymerase Chain Reaction (PCR). The resistance to colistin was determined with disc diffusion, E-test, and micro broth dilution method. According to the results of micro broth dilution as a gold standard, 43% of the isolates were resistant to colistin, while this percentage was 23% and 44% through E-test and disc diffusion methods, respectively. The positive and negative predictive value (PPV and NPV) of this method was 43% and 57%, respectively. The sensitivity and NPV index of E-test for the detection of colistin resistant strains was 76% and 68%. Detection of colistin MIC by E-test strips has been commonly used in clinical laboratories to recognize the colistin susceptible strains. The NPV and sensitivity of E-test method demonstrated that this method has inefficacy to accurate determination of colistin susceptible strains. Thus, using standard protocol micro broth dilution with qualified materials should be stabilized and replaced instead of disc diffusion or even using E-test in clinical laboratories.