Discordance in Colistin Susceptibility Test for Acinetobacter baumannii Showing Resistance: MicroScan versus Etest

Kang, Goeun; Kim, Hyun Soo; Kim, Han Sung; Song, Wonkeun; Kim, Jae Seok

Abstract

BACKGROUND
Acinetobacter baumannii causes various hospital-acquired infections, its multidrug resistance is rapidly increasing worldwide. Although colistin is used in treatments against multidrug-resistant A. baumannii, resistance to colistin has also been reported recently. Few studies have reported colistin susceptibility testing using MicroScan. In this study, we compared colistin susceptibility tests for resistant A. baumannii by MicroScan (Siemens, USA) and Etest (BioMerieux, France).
METHODS
We collected 115 A. baumannii clinical isolates, showing colistin resistance (minimum inhibitory concentration [MIC] > or =4 microg/mL) by MicroScan, from July 2014 to March 2015 at Kangdong Sacred Heart Hospital. Species identification and antimicrobial susceptibility tests were performed using the MicroScan Neg Combo Panel Type 72. Additionally, Etest was also performed for comparison.
RESULTS
Of the 115 isolates, Etest revealed that 103 (89.6%) were colistin-susceptible (MIC < or =I microg/mL). Moreover, 52 isolates, showing a MIC of 4 microg/mL by MicroScan, were all susceptible to colistin. Only 12 (19.0%) of 63 isolates, showing a MIC of >4 microg/mL by MicroScan were resistant to colistin according to the Etest.
CONCLUSIONS
The MicroScan automated system, using the commercial broth microdilution method, exhibited some discrepancies with the Etest for colistin susceptibility in A. baumannii. Therefore, more practical and reliable susceptibility tests for colistin are required in clinical laboratories using MicroScan.