Evaluation of a ceftazidime-avibactam and aztreonam disc diffusion synergy test using the broth disc elution method as the reference standard
DOI:
https://doi.org/10.61529/idjp.v35i3.576Abstract
Background: Ceftazidime-avibactam (CZA) combined with aztreonam (ATM) restores activity against metallo- -lactamase-producing Enterobacterales. Broth disc elution (BDE) is recommended for testing this combination but is often impractical in resource-limited settings. This study determined the diagnostic accuracy of a CZA–ATM disc diffusion synergy test using BDE as the reference standard.
Material and Methods: This prospective cross-sectional study was conducted at the Armed Forces Institute of Pathology, Rawalpindi, from November 17, 2025, to May 17, 2026. A total of 140 non-duplicate carbapenem-resistant Enterobacterales isolates non-susceptible to CZA and ATM were tested in parallel by disc diffusion synergy and BDE using independent, blinded readers. Sensitivity, specificity, predictive values, accuracy, and Cohen's with 95% confidence intervals (CIs) were calculated.
Results: BDE detected synergy in 80 of 140 isolates (57.1%). Disc diffusion yielded 55 true positives, 60 true negatives, 25 false negatives, and no false positives. This demonstrated a sensitivity of 68.8% (95% CI 57.9–77.9), specificity of 100% (95% CI 94.0–100), positive predictive value of 100% (95% CI 93.5–100), negative predictive value of 70.6% (95% CI 60.2–79.2), accuracy of 82.1% (95% CI 75.0–87.6), and Cohen's of 0.65 (95% CI 0.54–0.77).
Conclusion: The disc diffusion synergy test showed 100% specificity but moderate sensitivity for in-vitro CZA–ATM synergy. A positive result reliably identifies synergy, whereas approximately one-third of synergistic isolates were missed.
Keywords: Antimicrobial susceptibility testing, Aztreonam, Broth disc elution, Carbapenem-resistant Enterobacterales, Ceftazidime-avibactam, Disc diffusion, Metallo- -lactamase.
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