Multiplex Immunochromatographic Detection of OXA-48, KPC, and NDM Carbapenemases: Impact of Inoculum, Antibiotics, and Agar

For the rapid detection of carbapenemase-producingEnterobacteriaceae (CPE), immunochromatographic lateral flow tests (ICT) have recently been developed. The aim of this study was to assess the new multiplex ICT Resist-3 O.K.N. and to investigate if it can be performed directly from susceptibility testing plates. Additionally, the impact of the inoculum and carbapenem disks on sensitivity and specificity was evaluated. The new ICT was challenged using 63 carbapenem-resistantEnterobacteriaceae (CRE) isolates, including 51 carbapenemase producers. It was assessed under five different conditions directly from Mueller-Hinton agar (MHA): 1 μl or 10 μl of inoculum harvested in the absence of antibiotic pressure or 1 μl taken from the inhibition zone of either an ertapenem, imipenem, or meropenem disk. The sensitivity of the ICT was 100% for OXA-48-like and KPC carbapenemases and 94.4% for the NDM carbapenemase with the 1-μl inoculum. When harvested adjacent to a carbapenem disk, the sensitivity increased to 100%. Additionally, with zinc-supplemented MHA, both the sensitivity increased and the NDM band became visible faster (mean time, 8 ± 3.9 min for MHA compared to 1.9 ± 1.5 min for MHA plus zinc;P = 0.0016). The specificity of the ICT was 100%. The Resist-3 O.K.N. ICT is a sensitive and rapid test for the detection of three highly prevalent carbapenemases. However, false-negative results for NDM can occur. We recommend an inoculum of 1 μl that is harvested adjacent to an ertapenem or meropenem disk and the use of agars with sufficient zinc content to achieve the best performance.