Comparative Evaluation of Phenotypic Methods for Testing Ceftazidime-Avibactam and Aztreonam Synergy in Carbapenem-Resistant Enterobacterales from a Tertiary Care Center in Eastern India

Background Metallo-β-lactamase (MBL) producing carbapenem-resistant Enterobacterales (CRE) pose significant therapeutic challenges in clinical practice. The combination of ceftazidime-avibactam with aztreonam demonstrates promise against MBL-producing organisms, but standardized synergy testing methods remain inadequately defined. Objective To evaluate and compare the performance of different phenotypic methods for testing ceftazidime-avibactam and aztreonam synergy in clinical CRE isolates. Methods Fifty consecutive non-duplicate CRE isolates were collected from various clinical specimens at a tertiary care center in Bihar, India. Four phenotypic synergy testing methods were evaluated: E-test/disc method, double disc diffusion (discs placed 15mm apart), disc stacking method, and disc elution method. Carbapenemase genes were detected using multiplex PCR. Statistical analysis was performed using Fisher's exact test to compare method performance. Results Among 50 CRE isolates (26 E. coli, 24 K. pneumoniae), 37 (74%) demonstrated resistance to ceftazidime-avibactam. Synergy detection rates varied significantly between methods: E-test/disc method 27% (10/37), disc stacking method 78% (29/37), and disc elution method 81% (30/37). All isolates harbored NDM genes, with 30 (60%) co-producing OXA-48. The disc elution method showed significantly superior synergy detection compared to the E-test/disc method (p < 0.001). Conclusions The high prevalence of MBL genes among CRE isolates explains the limited efficacy of ceftazidime-avibactam monotherapy. The disc elution method demonstrated the highest synergy detection rate and offers a practical, reproducible approach for clinical microbiology laboratories.