Modified Hodge Test versus Indirect Carbapenemase Test: Prospective Evaluation of a Phenotypic Assay for Detection of Klebsiella pneumoniae Carbapenemase (KPC) in Enterobacteriaceae

Abstract

The currently recommended phenotypic test for the detection of carbapenemase-producing members of the family Enterobacteriaceae is the modified Hodge test (MHT). However, the MHT lacks specificity. Here we demonstrate an alternative phenotypic test, the indirect carbapenemase test, for the detection of bla KPC -producing isolates that has specificity superior to that of the MHT for non-Klebsiella Enterobacteriaceae. The last decade has witnessed the dramatic emergence and worldwide dissemination of carbapenem resistance in Enterobacteriaceae. In the United States, the most common mechanism of carbapenem resistance is the production of the Ambler class A serine ␤-lactamase Klebsiella pneumoniae carbapenemase (KPC) (1). The bla KPC gene product has the ability to hydrolyze all penicillins, cephalosporins, monobactams, and carbapenem antibiotics, which often leaves clinicians with few therapeutic options. As a consequence, infections due to carbapenemase-producing Enterobacteriaceae (CPE) carry a high mortality rate, reaching 40% or higher in some studies (2-5).The bla KPC gene is carried on resistance plasmids that are readily transmitted between bacterial strains and species, making the prevention of in-hospital transmission especially difficult (6-8). Accurate recognition of CPE in the clinical microbiology laboratory is central to controlling the spread of these organisms in the health care setting (9). However, identification of CPE from clinical isolates can be challenging. Molecular detection of a specific carbapenemase gene is the gold standard, but this method is impractical for many clinical laboratories.In 2009, the Clinical and Laboratory Standards Institute (CLSI) suggested phenotypic evaluation for the presence of a carbapenemase in Enterobacteriaceae with elevated MICs of one or more carbapenems by using the modified Hodge test (MHT) (9). The MHT has been found to be a useful tool for the detection of CPE but lacks specificity for serine carbapenemases (10-13). We previously reported a heterogeneous outbreak of KPC-producing Enterobacteriaceae (7,14). Starting in late 2009, there was an increase in Enterobacteriaceae isolates (mostly Enterobacter spp.) that were MHT positive but bla KPC PCR negative. This prompted us to prospectively evaluate phenotypic screening approaches for carbapenemase production. Here we compare the MHT and the indirect carbapenemase test (ICT), originally described by Moland et al. (15). Those investigators also described a direct carbapenemase test with imipenem, but we did not use it because some of our isolates expressed such high levels of resistance that no evaluable phenotypic result was possible.This study was conducted at the University of Virginia Medical Center, a 619-bed tertiary-care hospital in central Virginia, from 1 May 2010 to 31 December 2011. One hundred twenty-seven isolates of Enterobacteriaceae with ertapenem MICs of Ն1 g/ml were prospectively collected during the study period. Given the heterogeneous nature of our earlier outbreak (7), all species of En...

Evaluation of a New Phenotypic OXA-48 Disk Test for Differentiation of OXA-48 Carbapenemase-Producing Enterobacteriaceae Clinical Isolates

“…Moreover, although it is not within the scope of the present study, the test was able to distinguish KPC and/or VIM/KPC production effectively with a rather high sensitivity and specificity (Table 3). The sensitivity of the current indirect test was comparable with that of another indirect carbapenemase test, recently applied for the differentiation of KPC-producing Enterobacteriaceae in a university hospital in the United States (39). It should be mentioned that a disk test based on the use of EDTA to permeabilize the bacterial cell and release ␤-lactamase into the external environment was initially employed for the detection of plasmidic AmpC ␤-lactamases (40).…”

“…It should be mentioned that a disk test based on the use of EDTA to permeabilize the bacterial cell and release ␤-lactamase into the external environment was initially employed for the detection of plasmidic AmpC ␤-lactamases (40). Our approach differed from the test that employed one paper disk impregnated with EDTA, previously described for KPC detection (39), since we used a second disk impregnated with both EDTA and boronic acid in order to inhibit KPC production.…”

The Problem of Carbapenemase-Producing-Carbapenem-Resistant-Enterobacteriaceae Detection

“…The MHT also demonstrates good sensitivity for many other carbapenemases, including VIM, IMP, and OXA-48-like enzymes. For U.S. collections of Enterobacteriaceae, sensitivity has been documented between 93 and 98% (17,18). The test's performance in detection of NDM is generally recognized as much lower; in one study, only 7 of 14 NDM isolates were MHT positive (19).…”

Evaluation of the Carba NP Test in Oregon, 2013

“…None Ϫ Klebsiella pneumoniae (14) None Ϫ Serratia marcescens (2) None Ϫ Acinetobacter baumannii (4) None Ϫ a Fifty-nine of 66 total carbapenemase-producing isolates (89%) were positive by the Carba NP test (95% CI, 79 to 95%). The numbers by carbapenemase were as follows: 30/30 KPC isolates (100%), 17/17 NDM isolates (100%), 0/7 OXA-48-like isolates (0%), 4/4 VIM isolates (100%), 4/4 IMP isolates (100%), and 4/4 SME isolates (100%).…”