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Carbapenem Resistant Enterobacteriaceae (CRE) has become an increasingly recognized cause of infection during the past decade, likely because of the emergence of carbapenemase-producing strains. The ability to limit the spread of these pathogens will require effective laboratory methods to rapidly identify patients infected with these organisms.
The objective of this study was to evaluate the Carbapenem inactivation assay (Modified Hodge Test) as a confirmatory test for carbapenemase production among clinical isolates of Enterobacteriaceae.
Seventy samples of urine were collected during a month, from May 2011 to June 2011from patients with indwelling urinary catheters for more than 48 hours of catheterization (to reach a final number of Sixty one Gram negative rod isolates ). All patients were hospitalized in different ICUS of Ain Shams University and Kobry El kobba Military Hospitals. All urine samples were centrifuged, and then the deposit of each sample was inoculated onto the surface of different culture media; Blood, MacConkey’s agar and the new chromogenic Uriselect 4 media. All inoculated media were incubated aerobically at 37oC for 24 hours. The growing colonies were identified by standard microbiological techniques.
The sixty one isolates included in this study with Gram negative rod, morphology and negative oxidase test results were completely identified down to species level using the overnight ID 32 E strips. Thirty six isolates of them were E.coli (59.02%) while 25 isolates were Klebsiella pneumoniae (40.98%).
Twenty eight E.coli isolates (78%) among 36 E.coli isolates were identified as ESBL producers using the standard disk diffusion method. while a total number of 16 E.coli isolates (44.44%) were identified as ESBLs by disk approximation method.
Eleven Klebsiella pneumoniae isolates (44%) among 25 Klebsiella pneumoniae isolates were identified as ESBL producers using the disk diffusion method. while 15 Klebsiella pneumoniae isolates (60%) were identified as ESBLs by disk approximation method.
The comparison between the disc diffusion and the disc approximation methods for detection of ESBLs among the study isolates showed 62% rate of positive agreement.
As regards detection of CRE, disc diffusion method identified 18 isolates (11 E.coli and 7 K. pneumoniae) as resistant. The Sensitivity, Specificity, positive predictive value and negative predictive value of Disk diffusion test in correlation with MIC as a standard test for detection of CRE isolates were 100%, 86%, 61% and100% respectively.
All Enterobacteriaceae isolates were tested for carbapenemases production using The Modified Hodge Test. Eight (22.22%) among 36 E.coli isolates were positive for the Modified Hodge testing. While 3 out of the 25 Klebsiella pneumoniae isolates (12%) were positive for carbapenemases production detected by the Modified Hodge testing.
All isolates giving positive MHT were identified as being resistant to imipenem by tube dilution method (MIC ≥4 μg/mL).
MHT showed high performance quality in comparison to tube dilution method for detection of CRE (100% for sensitivity, specificity, PPV and NPV).