In-vitro Susceptibility Testing Methods for Ceftazidime-avibactam against Carbapenem-resistant Enterobacterales: Comparison with Reference Broth Microdilution Method

Introduction: beta-lactam antibiotics, mainly cephalosporins, and carbapenems, have been the mainstay of treatment for infections caused by Enterobacterales. However, their role in treating clinical infections has become limited because of the increase in resistance. There is a need to have cost-effective and rapid methods for antimicrobial susceptibility testing methods for newer antibiotics like ceftazidime-avibactam against carbapenem-resistant Enterobacterales (CRE), which can be applied in routine clinical microbiology laboratories. With this aim, the present study was conducted to compare the disk diffusion and gradient diffusion, i.e., the E-test method with the reference broth microdilution (BMD) method for in-vitro testing of ceftazidime-avibactam against CRE. Materials and Methods: A total of 111 CRE isolates from various clinical samples were included. Conventional PCR (Polymerase Chain Reaction) was done for the detection of genes encoding carbapenemases and to see their expression, modified carbapenem inactivation method (mCIM) along with EDTA (Ethylenediaminetetraacetic acid) carbapenem inactivation method (eCIM) was done. Results: 42.3% (47/111) isolates were resistant to ceftazidime-avibactam by the standard broth microdilution method; however, 45.9% (51/111) were resistant by both disk diffusion and E-test. In 5.4% of isolates (similar in both methods), microbroth dilution method results did not match with E-strip and disk diffusion. Very major errors (VME) by both disk diffusion and E-test were found in 2.1% (1/47), and major errors (ME) were found in 7.8% (5/64) isolates (similar isolates in both methods). The overall categorical agreement (CA) rate was 94.6% for both E-test and disk diffusion, and the essential agreement (EA) rate was 90.1% (100/111) for E-test. 98% (109/111) of CRE harbored carbapenemase genes either singly (30.3%) or in combination with others (69.7%). Conclusion: In conclusion, for CRE, E-test and the disk diffusion method for ceftazidime-avibactam depicted an acceptable performance as an alternative to the reference broth microdilution method.