Comparative Evaluation of VITEK® 2 AST-XN21, CHROMID® Colistin Agar, and Reference Broth Microdilution for Colistin Susceptibility Testing in Carbapenem-Resistant Acinetobacter baumannii and Klebsiella spp.

Rapid and accurate testing of colistin resistance is of paramount importance for the effective management of gram-negative infections in workflow laboratories. The objective of this study was to evaluate the performance of commercial VITEK®2 AST-XN21 cards and ChromID media via the reference Diagnostics MIC-COL method. Colistin susceptibility testing was conducted on 72 nonduplicate isolates using Diagnostics MIC-COL as the reference method, in comparison with the VITEK®2 AST-XN21 card and ChromID agar. The colistin resistance rate was 37.5% (27/72) according to the Diagnostics MIC-COL (Klebsiella spp. 34.4% (n=10), and Acinetobacter baumannii (A. baumannii) 39.5% (n=17)), whereas the VITEK®2 AST-XN21 card resistance rate was 41.3% (n=12) for Klebsiella spp. and 48.8% (n=21) for A. baumannii. The VITEK®2 AST-XN21 card was CA 91.6%, and the very major error (VME) and major error (ME) rates for all the isolates were 1.38% (n=1) and 6.9% (n=5), respectively. A. baumannii isolates were CA (37/43) 86%, VME (1/43) 2.3% and ME (5/43) 11.6% with XN21 cards. For Klebsiella spp., CA (29/29) was 100%, whereas ME and VME were not detected. The CA was 94.4% (68/72) for ChromID medium, 93% (40/43) for A. baumannii and 96.5% (28/29) for Klebsiella spp. These results indicated that the CHROMID medium is an appropriate method for the screening of colistin resistance. Nevertheless, it is imperative to exercise caution when interpreting the results of a commercial automated VITEK®2 AST-N21 card for colistin susceptibility testing.