Further Modification of the Modified Hodge Test for Detecting Metallo-β-Lactamase-Producing Carbapenem-Resistant Enterobacteriaceae.
BACKGROUND
The modified Hodge test (MHT) was designed to detect carbapenemase-producing Enterobacteriaceae (CPE). This study evaluated variables to improve the performance of MHT.


METHODS
Carbapenem-resistant Enterobacteriaceae isolated from November 2010 to March 2013 at the Asan Medical Center, were evaluated, including 33 metallo-β-lactamase (MBL) producers and 103 non-CPEs. MHT was performed by using two carbapenem disks (ertapenem and meropenem; Becton Dickinson, USA), three media (Mueller-Hinton agar (MHA), MacConkey agar (MAC), and zinc-enriched MHA), and two inoculums (0.5-McFarland [McF] suspension and a 10-fold dilution of it.) PCR was performed to detect β-lactamase genes of the MBL, AmpC, and CTX-M types.


RESULTS
The sensitivity of MHT for detecting New Delhi metallo-β-lactamase (NDM) producers was highest using ertapenem and 0.5-McF, 52.0% on MHA and 68.0% on MAC, respectively. NDM-producing Klebsiella pneumoniae (NDMKP) were detected with higher sensitivity on MAC (78.6%) vs. MHA (28.6%) (P=0.016), but VIM-producing Enterobacter, Citrobacter, and Serratia were detected with higher sensitivity on MHA (78.5%) vs. MAC (14.3%) (P=0.004). MBL producers were consistently identified with lower sensitivity using meropenem vs. ertapenem, 39.4% vs. 60.6% (P=0.0156), respectively. The effects of zinc and inoculum size were insignificant. Enterobacter aerogenes producing unspecified AmpC frequently demonstrated false positives, 66.7% with ertapenem and 22.2% with meropenem.


CONCLUSIONS
The MHT should be adjusted for the local distribution of species and the carbapenemase type of MBL producers. MAC and ertapenem are preferable for assessing NDMKP, but MHA is better for VIM. Laboratory physicians should be aware of the limited sensitivity of MHT and its relatively high false-positive rate.
Department of Laboratory Medicine, University of Ulsan College of Medicine and Asan Medical Center, Seoul, Korea.
Hyun-Ki
HK
Kim
Department of Laboratory Medicine, University of Ulsan College of Medicine and Asan Medical Center, Seoul, Korea.
Jeong Su
JS
Park
Department of Laboratory Medicine, University of Ulsan College of Medicine and Asan Medical Center, Seoul, Korea.
Heungsup
H
Sung
Department of Laboratory Medicine, University of Ulsan College of Medicine and Asan Medical Center, Seoul, Korea.
Mi-Na
MN
Kim
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D015780:Carbapenems / Q000494:pharmacology; D004269:DNA, Bacterial / Q000235:genetics / Q000378:metabolism; D052978:Disk Diffusion Antimicrobial Tests; D024881:Drug Resistance, Bacterial; D004755:Enterobacteriaceae / Q000187:drug effects / Q000201:enzymology* / Q000302:isolation & purification; D004756:Enterobacteriaceae Infections / Q000382:microbiology; D006801:Humans; D060885:Multiplex Polymerase Chain Reaction; D010641:Phenotype; D001618:beta-Lactamases / Q000235:genetics / Q000378:metabolism*
D000900:Anti-Bacterial Agents; D015780:Carbapenems; D004269:DNA, Bacterial; D001618:beta-Lactamases; C552864:beta-lactamase NDM-1
Carbapenemase; Enterobacteriaceae; Klebsiella pneumoniae; Metallo-β-lactamase; Modified Hodge test