Performance of ceftazidime/avibactam susceptibility testing methods against clinically relevant Gram-negative...

Area of Science:

• Clinical microbiology
• Antimicrobial susceptibility testing

Background:

• Accurate susceptibility testing is crucial for guiding antibiotic treatment.
• Ceftazidime/avibactam is an important antibiotic for treating infections caused by Gram-negative organisms.
• Evaluating different susceptibility testing methods ensures reliable results.

Purpose of the Study:

• To evaluate the accuracy of Etest and disc diffusion methods for ceftazidime/avibactam susceptibility testing.
• To compare these methods against the reference broth microdilution (BMD) method.
• To determine optimal disc diffusion thresholds for ceftazidime/avibactam.

Main Methods:

• The study evaluated Etest, 30/20 μg disc, and 10/4 μg disc methods against BMD.
• 102 Gram-negative isolates were tested, including ceftazidime- and meropenem-resistant Klebsiella pneumoniae and multidrug-resistant (MDR) non-K. pneumoniae.
• Essential and categorical agreement, along with error rates, were determined per CLSI guidelines.

Main Results:

• 78% of isolates were susceptible to ceftazidime/avibactam.
• The Etest showed the highest categorical agreement (95%) with minimal major errors (6.3%).
• Against resistant K. pneumoniae, Etest and the 10/4 μg disc met equivalency; none met it for MDR isolates. A susceptibility threshold of ≤19 mm was identified for 30/20 μg discs.

Conclusions:

• Etest is a suitable alternative to BMD for testing ceftazidime/avibactam against resistant K. pneumoniae.
• The 30/20 μg discs may overestimate resistance, potentially leading to suboptimal treatment choices.
• Further optimization of disc diffusion methods is needed for MDR Gram-negative organisms.