Evaluation of MASTDISCS AmpC & ESBL ID Set for accuracy of rapid identification

Abstract

Introduction: The increasing prevalence of Enterobacteriaceae resistant to third-generation cephalosporins poses a significant global health concern. Accurate and timely detection of resistance determinants is essential for preventing hospital-acquired infections and supporting antimicrobial stewardship. This study evaluated the diagnostic accuracy of the AmpC/ESBL Detection Discs (Kanto Chemical Co., Inc.) in differentiating ESBL and AmpC producing isolates.Methods: Sixty-nine Enterobacteriaceae strains were analyzed. Disc diffusion profiles after 4, 6, 8, and 18–24 hours of incubation were compared with molecular results obtained by polymerase chain reaction (PCR) or next-generation sequencing (NGS). Additional susceptibility testing with cloxacillin was performed to clarify discordant cases. Results: No interpretable growth was observed at 4 hours. For ESBL producers, concordance was 57.6%, 66.7%, and 66.7% at 6, 8, and 18–24 hours, respectively. For AmpC producers, the corresponding values were 33.3%, 100%, and 91.7%. Isolates with outer membrane protein loss showed reduced agreement (20%, 80%, 40%), while carbapenemase-producing Enterobacteriaceae displayed similarly low consistency (47.1%, 76.5%, 76.5%). Notably, isolates harboring blaCTX-M-9 demonstrated inhibition zones in the presence of cloxacillin, leading to misclassification as AmpC producers. Conclusion: The 8-hour protocol yielded performance comparable to the conventional 18–24-hour method, suggesting practical utility in routine diagnostics. However, cloxacillin interference in blaCTX-M-9 isolates represents a limitation requiring careful interpretation. Given its affordability and operational simplicity, this approach may serve as a feasible rapid screening strategy in diverse clinical laboratory settings.