271. Do CLSI breakpoints Predict Presence of Klebsiella pneumoniae Carbapenemase (KPC) Gene among USA Enterobacteriaceae isolates?
Session: Poster Abstract Session: Antimicrobial Susceptibility Testing
Friday, October 21, 2011: 1:35 PM
Room: Poster Hall B1


The 2010 CLSI guidelines lowered susceptibility breakpoints for carbapenems to improve detection of carbapenemase producing resistance among Enterobacteriaceae (ENT). Breakpoints were added for doripenem (DORI). We evaluated the frequency of carbapenemase genes among common ENT and impact of the new breakpoints for detecting KPN harboring KPC.


K. pneumoniae (KPN; n=2049), K. oxytoca (KOX; 366), E. cloacae (ECC; 974), and E.coli (EC; 3481) isolates were collected from 26-42 USA medical centers from 2007-2009. Susceptibility testing was performed by CLSI broth microdilution method. Isolates with imipenem (IMP), meropenem (MERO), and/or ertapenem (ERT) MIC ≥ 2 g/ml (n=161) and selected isolates with IMP MIC ≤ 1 g/ml (n=74) were screened by PCR for eight carbapenemase genes (blaIMP, blaVIM, blaKPC, blaSME, blaGES, blaIMI, blaNMC-A, blaOXA-48).


Of 235 isolates screened by PCR, 137 contained blaKPC; no other carbapenemase genes were detected. KPN (n=112, 5.5%) accounted for the vast majority of KPC+ organisms, followed by ECL (n=18, 1.8%), KOX (n=6, 1.6%), and EC (n=1, 0.03%). The MIC ranges (g/ml) for KPC+ KPN (112 isolates) were 0.5 - >8 for IMP and MERO (MIC50, >8 and MIC90, >8), 2 - >8 for ERT (MIC50, >8 and MIC90, >8), and 0.5 - >8 for DORI (MIC50 8, MIC90 >8). Among KPC+ KPN, the sensitivities of original and updated breakpoints were high for all carbapenems, 96-99% and 99-100% respectively. Specificities for original and updated breakpoints were 87-99% compared to 82-89%. ERT had highest sensitivity for both original (99%) and updated (100%) breakpoints, but the lowest specificity (87% and 82%, respectively).


The KPC gene was most common among ENT; no other carbapenamase genes were detected. Overall sensitivity was high for detection of KPC+ KPN and increased by 1-3% when applying updated CLSI susceptibility breakpoints, while specificity decreased by 5-10%. The lower number of false negatives should improve antimicrobial therapy and enhance the efficacy of infection control for patients with KPC+ KPN. However false positive results, particularly with ERT, may unnecessarily restrict carbapenem therapy.


Subject Category: A. Antimicrobial agents and Resistance

Robyn M. Kaiser, MD, MPH, Univ. of Minnesota Med. Ctr., Minneapolis, MN, David J. Farrell, Ph.D., Microbiology, JMI Laboratories, North Liberty, IA, Matthew Stilwell, BS, JMI Lab., North Liberty, IA, Fred Tenover, Ph.D. D(ABMM), Antibiotic Resistance Working Group, IDSA, Sunnyvale, CA and Ruth Lynfield, MD, Antibiotic Resistance Working Group, IDSA, St. Paul, MN


R. M. Kaiser, None

D. J. Farrell, None

M. Stilwell, None

F. Tenover, None

R. Lynfield, None

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