Evaluation of resazurin-based rapid test to detect colistin resistance in Acinetobacter baumannii isolates

Acinetobacter baumannii primarily causes colonization, yet it can be an opportunistic pathogen associated with hospital-acquired infections. Many countries report rapid spread of carbapenem-resistant Acinetobacter baumannii (CRAb) which limits treatment options, with colistin frequently being the last line treatment option. The aim of our study was to evaluate a recently developed rapid method, namely the Rapid ResaPolymyxin test, for detection of colistin resistance (ColR) in Acinetobacter baumannii . This test was used for rapid screening of colistin resistance in a clinical setting where there is endemicity of CRAb isolates. A total of 82 A. baumannii clinical isolates were included in the evaluation. The majority of them were resistant to carbapenems (75/82, 91.5%). A total of 37 isolates (45.1%) were resistant to colistin, all being resistant to carbapenems. None of the ColR isolates carried the plasmid-mediated mcr-1 to -5 genes. The Rapid ResaPolymyxin NP test reached a 95.1% categorical agreement with results of reference broth microdilution method, with 93.3% sensitivity and specificity, and positive and negative predictive values being respectively at 92.3% and 97.7%. The Rapid ResaPolymyxin NP test performed well on our collection of clinical and surveillance CRAb isolates from the Central Slovenia region. The test is inexpensive and easy to integrate into laboratory workflow. The main value of the test is rapid categorization of susceptibility and resistance which has important implications with respect to the treatment strategy as well as the infection control measures.