Carbapenem resistance in Klebsielia pneumoniae not detected by automated susceptibility testing

Detecting P-lactamase-mediated carbapenem resistance among Klebsiella pneumoniae isolates and other Enterobacteriaceae is an emerging problem. In this study, 15 b/a(KPC)-positive Klebsiella pneumoniae that showed discrepant results for imipenem and meropenem from 4 New York City hospitals were characterized by isoelectric focusing; broth microdilution (BMD); disk diffusion (DD); and MicroScan, Phoenix, Sensititre, VITEK, and VITEK 2 automated systems. All 15 isolates were either intermediate or resistant to imipenem and meropenem by BMD; 1 was susceptible to imipenem by DID. MicroScan and Phoenix reported 1 (6.7%) and 2 (13.3%) isolates, respectively, as imipenem susceptible. VITEK and VITEK 2 reported 10 (67%) and 5 (33%) isolates, respectively, as imipenem susceptible. By Sensititre, 13 (87%) isolates were susceptible to imipenem, and 12 (80%) were susceptible to meropenem. The VITEK 2 Advanced Expert System changed 2 imipenem MIC results from >= 16 mu g/mL to <= 2 mu g/mL but kept the interpretation as resistant. The recognition of carbapenem-resistant K. pneumoniae continues to challenge automated suscepibility systems.