An update on polymyxin susceptibility testing methods for Acinetobacter baumannii.

Introduction: Carbapenem-resistant Acinetobacter baumannii (CRAB) currently comprises >50% of A. baumannii isolates in Europe and the USA and commonly are extensively or pandrug resistant. They cause severe hospital infections, which according to CDC have very high mortality. Polymyxins are universally recognized as the last-line choice against CRAB infections. The role of polymyxin susceptibility testing (ST) for these pathogens remains a subject of debate, although it is critical for selection of appropriate antimicrobial therapy. There are significant discrepancies between in vitro ST methods and important shortcomings for diffusion and automated methods have been repeatedly verified. The joint CLSI-EUCAST Working Group has recently recommended broth microdilution (BMD) without additives as the reference method for polymyxin ST. Areas covered: This review, focused on this threatening pathogen, summarizes the current available ST methods for polymyxins and discusses the challenges encountered by clinical microbiological laboratories in obtaining and interpreting valid susceptibility results. Studies on polymyxin ST methods for Acinetobacter spp., retrieved from Pubmed database until May 2019, were evaluated. Expert opinion: Currently, no single commercial ST method for polymyxins is providing fully accurate results. Further optimization and standardization of the ST methods and international harmonization of guidelines should be achieved to facilitate the accurate detection of polymyxin-resistant A. baumannii.